The AM Forum

I ran a series of 80m tests to-day with the 440Ω untuned feedline between shack and balanced tuner at the tower, and compared to the 50Ω coax I have used for years.  The coax is 1 y.o. RG-213 elevated off the ground, using the same poles as the OWL.

The OWL is matched to the link-coupled transmitter final using a balanced L-network.  The coax is matched using a series variable capacitor between the link and the coax. In each case, the transmitter was  loaded up to precisely 120 watts DC input.

At the tower end, a balanced link-coupled tuner is used to feed the tuned open-wire transmission line that goes up the tower to feed the dipole. A thermocouple RF ammeter was inserted in one side of the OWL tuned feeder to measure the relative output from the balanced tuner to the tuned feeders that go to the  dipole.  Readings were taken at the extreme ends and the middle of the band.

With the 50Ω coax, a 3-turn link was used, tuned to resonance with approximately 250pf of capacitance in series with the  link. With the 440Ω OWL, 9, 9 1/2 and 10 turns were tried with the link, with no series capacitor.  The best match across the entire band was achieved with the 9-turn link, but the rf output reading was close to the same with all three links, once the transmitter was adjusted to exactly 120 watts input. With both the coax and OWL, reflected power was nulled with the tuner to barely perceptible readings.

Here are the results with the OWL:

3500 kc/s   0.76 Amps
3750          0.79
4000          0.71

With the 50Ω coax:

3500 kc/s   0.673 Amps
3750          0.71
4000          0.65

In each case, with the same DC input, the rf current to the dipole's tuned feeders using the coax transmission line from the shack was roughly 88 to 91% of the reading using the untuned 440Ω OWL.  That means the power delivered by the coax was only about 80% of the power delivered by the OWL.

Assuming 65% efficiency from the transmitter and tuners, the tuner is looking into roughly an average load of 135Ω across the band, as seen at the bottom end of the half-wave tuned feeder, +/-  j factor.

Don,

I have always liked and used OWL for my transmission lines. The one exception is that I feed my verticals with coax, but loops and dipoles always get OWL. In the past, I have had a few transmitters that use the swinging link output arrangement, and I have always attached COAX to the swinging link, and the coax would then go to a balance tuner in the shack, and from there to the antenna would be the OWL. I have never used any sort of tuner external to the shack. Because of the properties of OWL, I have never felt the need, and my antennas have always seemed to work adequately. Your post has made me think that I could probably get better performance from my OWL fed antennas.

A few questions come to mind: 1. The shield of a coax attached to the swinging link at the transmitter end is at DC ground potential. Is some of that RF power actually lost to the ground? Obviously, RF energy acts very differently than DC energy, but I would think that some of that RF energy is heating dirt.

2. With your system, could you have eliminated the balanced L network at the shack end, and just run the OWL to the external tuner? Do you think this would have degraded the performance of the antenna system?

3. Alternatively, could you have eliminated the outdoor tuner?

4. In your test, where do you think the power is lost? In the coax itself? in plate circuit heat? a combination thereof?

5. I assume that "tuned feeders" at the tower are "tuned" by virtue of their length, perhaps I am wrong about that, but if so, they would be "tuned" at only one frequency (or a small bandwidth of frequencies). Is this a single band antenna system?

One of my recent acquisitions is a large transmitter using a swinging link output circuit. I had the notion that I might just run OWL all the way from the transmitter to the antenna with one tuner positioned somewhere in between. Also, if you have the time, it would be interesting to see schematics of your two tuners.

Ron

I think the coax is grounded at the transmitter, but I didn't make it a point to do so. It is not grounded at the tower end; it, the  link and the series capacitor are all left floating. I think the loss is cumulative, divided between the link at  the transmitter end, the coax and the link at the tower end. I have run a measurement comparing the output to the input of the line only, and the coax was 93% efficient while the OWL was 98%

The OWL is run without standing waves  between shack and tower.  I had to build a matching network to match the link to the line, and the L-network worked best.  I tried several designs.  With the coax, the series resonating capacitor matched it OK.  The OWL is  connected to the external tuner directly to the 9-turn link, with no resonating capacitor on that end.  If you get the link just right, it will resonate without an external  capacitor.  Without the matching network at each end, the OWL would  run with standing waves, or wouldn't take a load.

That would make it a tuned line all the way from the dipole to the shack.  The problem is the length. The frequency error over each quarter-wave of tuned feeder is cumulative.  Too many quarter waves, and the tuning becomes sharp and critical with limited bandwidth.  Also, on 80m, I would likely have to switch from series feed to parallel feed at some point while QSYing across the band.  Keeping the line flat from shack to tower, and running the tuned line up the tower limits the number of tuned quarter-waves to two, and a simple balanced tuner with only one variable element can handle that. Since it is remotely tuned from the shack using a reversible motor, I am striving to limit the variable element at each tuner at the tower (have 5 of them ganged  togetther for various bands and antennas) to one variable capacitor. This already works with the  coax, so it is merely a matter of adapting each tuner to be fed with 440Ω balanced line instead of 50Ω unbalanced.

See no. 1


It is multi-band.  I work 160-40m using that same dipole and tuned feeder.  The lengths are so that it uses series tuning on 80 and parallel tuning on 40.  On 160 I switch in an additional 60 ft. of resonant feeder to get an even number of 1/8 wavelengths, and parallel tune it.

I also feed the tower, which is base insulated, as a quarter-wave vertical on 160.  I rarely use the dipole on 160, but that was the only antenna I had for the band when I first built the tower, before installing a radial system. I never wasted my time trying to load up the vertical with no ground system.

I'll try to post some diagrams as soon as I get the whole thing operating.

The meaning of "tuned feeders" has changed over the years I think.  From what I have read in my 1930s Handbooks (ARRL) the term at that time referred to balanced flat lines engineered to match the load Z at some frequency by way of length and spacing.  I now hear the term used to refer to any balanced line with a balanced matching network.

I went to one of the popular on-line loss calculators http://www.ve3sqb.com/hamaerials/ocarc/

and put in Belden 213, 120 feet flat 50 ohms 1:1 vswr 1 KW in on 3500 kc and the calculated loss was 100 w. (900 making it to the other end).  .435 dB loss.

Generic 600 ohm ladder line:  same power, length, 1:1 vswr flat line, 3500 kc loss .043 dB -- 990 watts making it to the other end.

this is why I avoid long runs of RG213, but it is great for jumpers and 5 foot runs.

What about something like 1/2 inch heliax.   

For LDF4-50,  all same condx just different feedline, we do a little better but not as good as OWL:  967 watts at the load end, .145 dB loss. 

Caveat: I don't know how the on-line calculator works i.e. what the equations it uses are and without that, you have to take this data with some precaution.  However, it agrees with Don to the extent that OWL is indeed the winner  :D

Don...does rain , snow, or ice make much tuning difference with your setup ? Does it snow in TN? Keep up the good work...Steve

We get it all here.  Rain, snow, ice, fog, hail, sleet, etc.  No snow or ice this year, but this could be called the year without a winter. Fortunately, ice doesn't occur too often, maybe a light icing a few times per winter.  Every decade or two we get a major ice storm that takes down power lines, trees and antennas.

Ice is the worst about detuning the antenna.  I can usually re-adjust the tuner and make it match well enough to get on the air, but a couple of times  when we  had heavy ice (fortunately the antenna held up) I couldn't get the transmitter to take a load no matter what, and just had to wait till it melted.  Rain and fog sometimes detune it enough that I have to re-set the tuner away from normal settings, but it works OK until it dries, then I have to re-tune to normal settings.  Dry snow doesn't seem to affect it much, but heavy wet stuff that accumulates on the OWL spacers sometimes detunes it much the same as light ice.

Rob, I believe the term "tuned feeders" has always meant knowingly running a transmission line with a significant standing wave. I would consider anything running a VSWR greater than about 3:1 as tuned feeders, but I'm not sure what the industry standard is, if there is one.  Untuned theoretically means the transmission line is running perfectly flat with no standing waves, but in actual practice most "untuned" transmission lines regardless of type (coax, OWL, "window" line, solid ribbon, etc) show some reflected power, but the transmatch, tuner or output network in the transmitter is able to compensate for the reactive component and mismatch, if it isn't too great. Real OWL makes the most efficient tuned feeders.

You don't necessarily have to use a full-fledged tuner with open wire tuned feeders.  For instance, an end-fed zepp with a quarter-wave feeder will show low impedance at the transmitter end of the feeders. If the feeders are cut to just the right length, a link coupled transmitter can be made to see exactly the capacitive reactance needed to tune the link to resonance; the transmitter will take a load and the antenna will work great within a narrow range of frequencies.  To make that antenna work across the entire band, cut the feed line slightly short and insert a variable inductor in series with the link.  The inductor will serve to tune the system to resonance.  Or you can make the feeders a little too long, and tune to resonance with nothing more than a variable capacitor in series with the link, or preferably, two variable capacitors, one in series with each side of the link.

A couple of long-time Hammy Hambone misconceptions include the old wives' (or husbands'?) tales that the reflected power comes back into the transmitter and heats up the final tubes sometimes causing them to run red, and that you can "trim" the length of the coax to get the SWR down.  If you have to trim the feed line to get the antenna to load, you are in fact running a tuned feeder and adjusting its length for the best match the transmitter output network can handle, as described in the previous paragraph. Trimming the length has no effect whatever on the SWR on the feed line.

Too many of the antenna discussions I hear over the air, especially those using the "other mode", sound something like this (http://www.youtube.com/watch?v=zOOihiLLS4I&feature=related).  ;D

Try the OWL again during a good rain both with no retuning and with.

The answer to coax loss is to use CATV hardline....mo betta in long runs.

Running any OWL here except maybe a 70' run of light window line to an 80M sloper would be about impossible and that crap goes bonkers in the rain....tried it decades ago.

My #10 spaced 4 inches with Johnson spreaders might need a slight tweek if the line is iced up otherwise twice a year. When the leaves come out and when the leaves drop. Rain doesn't do anything

Not surprising.  That stuff is nothing more than heavy duty TV ribbon with little rectangular holes punched in the dielectric.
 
There was also a type of coax that consisted of copper tubing as the outer conductor, and the inner conductor was solid copper wire with little ceramic beads crimped on every few inches.  The rest of the dielectric was air.  In should  be almost as efficient as OWL, if you can keep the moisture out.  Broadcast stations would slightly pressurise the line with air, or preferably, with dry nitrogen.  Don't know if it is still in production or not.

This is correct.  I went and looked in my 1932 ARRL handbook.  I got it exactly the opposite.  An untuned feeder is a flat line.  A tuned line has standing waves on it.   Sorry about that.

There is a 5kw 3-tower DA here in Bowling Green, Ky that uses the circa 1948 line you mention.  The center is actually copper tubing too, and I believe it's some funky impedance like 64 ohms.  I spliced it a few years ago after a mowing accident.

BUT, if you really want to tune coaxial cable from the shack with an unbalanced tuner, this coax will rival the low loss of open wire line even at high VSWR:

http://w4neq.com/img/low_loss_coax.jpg

Chris

I use the "brown crappy" window feedline.    It is the heavy duty made with #14 wire.    The rain is no problem on 80M, a slight change on 40M, the upper bands the tuning goes wild.  It has been durable, however, the antenna and feedline have been up at least 15 years.  Will change to OWL at the next opportunity.

That's just 3 or 4 inch heliax.  It won't rival OWL in terms of the loss to your bank account in $/foot plus the nitrogen and associated hardware needed to pressurize it once you get done paying for it.

Actually, it's Andrew HJ9-50 - five-inch Heliax.    At 2 MHz  it loses 0.011 dB per 100 feet and will handle 633 kw average power.  When I bought some last, it was running about $60 per foot.

Personally, just by sheer coincidence, on 80 / 40 I run the same OWL as Don - #10 bare copper spaced 2 inches.

The only real disadvantage of tuned feeders is that with really long runs the bandwidth gets pretty narrow, so re-tuning with QSY must be done from a look-up table.

I have always thought about using tuned feeders for a 40 / 80 ground plane up above the trees.  But I've never been able to come up with a suitable balun design to do it over a wide bandwidth with low loss.

Chris

Trilogy makes a mostly air CATV coax, Im running 200' of their 1" to the 144 and 222 antennas, all from local installers surplus.
The 7/8" goes to the 10M stacks which fell down in an ice storm.

I also have some of that old 50 Ohm stuff but it requires pressurization and is long useless by now. It will go to a scrap dealer when copper peaks.

That's true for anything with copper in it.  But the price of copper products seems to be rising at a faster rate than  the price of copper, not unlike the price rise in university tuition and medical care, compared to the rate of inflation.  Yesterday, I was at Lowe's for something else, and went over and took a look at what they are charging for ordinary house wiring.  Incredible! Sticker shock!  Several times the price per foot that I paid last time I bought some. This morning I looked in the commodities section of the local newspaper. To-day's price (NY Merc) is $3.8790/lb.  A few years ago, as I recall, it was about half that, but the price of copper wire in the store was probably less than a quarter what they are selling it for now.

Go to garage sales for wire, especially the ones where people are moving out of their homes to retirement places.    I just got a spool of zip cord that had an original price of $29 marked on it.    I think I paid $1 or less for it.

I think when I was putting in my ground system, I was buying those 500' rolls of insulated no. 14 solid at home depot because they had the best price, $14 / roll.  I think they're around $45 now. I saw last time I was there that they now sell 100 foot rolls, part of the trick to make it seem cheaper.  Since I wasn't looking for 20,000 or 30,000 feet I settled for the 500 foot rolls and discovered that the wire with insulation was cheaper than the wire without, I guess because uninsulated no. 14 is odd and there is economy of scale with the insulated.  The thing that is a downer is that it was hard before to persuade some hams to invest in a good ground system, but with the price of wire today it's about impossible.

What I miss are the wide spaced insulators like what you can see advertised in Handbooks and QST from the early 1950s.  I wish someone still made those pyrex (or whatever it was) thin lightweight 6 inch spacing ladder line spacers.  

If wire with plastic insulation is cheaper, then use it.  It won't make any difference in performance, but it will protect the wire from minerals in the soil and the radials will last longer.  That's not a problem in our soil here, but in areas with highly acidic soil, bare copper will gradually erode away. The kind of plastic insulation they use now should remain intact in the soil for decades, if not centuries.

But still, I can't see why bare wire should be more expensive than insulated.  The companies that manufacture the insulated wire buy the bulk stock in bare form from other manufacturers, who in turn bought the copper in ingot form, melted it down and extruded the wire from the molten mass. These vendors don't sell to the general public, but only to the manufacturers that make the finished product, at the prevailing copper price plus their "extruding fee".  It would be cheaper for the finished product manufactures to simply reel out wire from their stock and sell to customers as-is with  some mark-up, than to go to the trouble of adding insulation to the wire before selling it. That's how I got my copper wire for the radial system; my old room mate's father ran such a wire manufacturing plant in RI, and he sold me 16,000 feet of #12 softdrawn bare at his cost.  I paid $400 total, for the wire in 4 spools back in 1974.  It wasn't cheap even back then, but quite a bit less than what it would have cost at the local electrical supply retailer. The guy who owned the factory and sold me the wire thought I was totally freakin' nuts when his son told him I planned to bury all that wire in the ground.

When I had cut the 16,000 ft. of wire into 120 pieces, each 133' 4" long, I bundled the pieces all together and laid them out on the ground before installing them into the ground system.  The solid bundle was between 4" to 6" in diameter and 133' 6" long. Wouldn't that have made to-day's copper thieves drool!  And the wire sat out openly in the field unguarded for several weeks before I got it all laid out and buried.

Not much new here.

Window line sucks when wet.

Coax sucks with high SWR.

Open wire line sucks to install properly.

All can work just great if people understand their limitations.

Maybe there's no logic--they just set a price and since it is unusual, if someone wants it, they must want it bad enough to pay the price.

Or, there's a small market for it (no. 14 bare for example)  but the shipping and plastic spools and inventory management and labeling and wrapping and so on all cost just as much but fewer buyers are paying for that so it is more.  Just speculating.

Since I did not have room for a complete ground system in all directions, I drew a map of my lot with all the obstructions on it and mapped out where the radials would go and how I would angle them to stay inside my lot but parallel to the horizontal part of the inverted L over head.  I drew quadrants around the feedpoint and subdivided them on the map.  I knew if I just went out there and started laying down wire willy nilly I'd get about halfway done and discover I'd screwed it up so I made a plan.  Then I went out there and strung out radials two at a time each hand holding a wire spool dividing up a quadrant until it was full.  Walking along on the ground I'd follow the plan and when I had reached the end, marked by an obstruction, lot line or 120 foot limit I'd cut the wire and walk back with a bucket of staples and a hammer and spaple them down into the ground.  This was in Nov. 2003.  I worked on weekends and for about 90 minutes after work on weekdays before it got dark.  It took about three weeks but I eventually got 101 radials down.   I was going for 120 but I decided no one would notice them on my signal and that wire might be more useful for other things.  I used no. 14 solid insulated.  I think I spent about $90 on wire.   the vertical wire is bare no. 14 hard drawn stranded.   

I haven't checked Wire Man lately, and compared their prices for bare copper wire to what I could get at the local big boxes or at a dedicated electrical supplier.  I have bought from them at hamfests, but they usually don't bring along their complete line of products.

Something I find baffling is that one particular size of solid copper wire is extremely hard to find.  It is easy to find #'s 18, 16, 14, 12, 10, 6 and 4, but for some reason nobody wants to stock #8, which I find useful for coil stock, as well as for constructing OWL. Fortunately I have the big roll of #8 copperweld, which seems to work OK for my untuned transmission line, but the skin depth is small enough that I'd be a little leery of using it with high SWR.  I would want solid copper for that. Also, odd number wire sizes are almost non-existent, except at motor re-winding shops. OTOH, at fence suppliers, some odd number steel wire sizes are readily available.

At one time #8 solid was a standard house ground. Maybe there is a conspiracy to force #6 to be used by everyone including Harry Homeowner, the weekend electrician and his cousin Hammy Hambone. I still have a partial reel of #8 solid copper I bought around 1975

Data has been collected for the 160m dipole configuration.  The dipole is approximately 135' long, fed on 160m with a total of 186' of tuned feedline.  Thermocouple rf ammeter was placed at the output terminals at the feed point of the tuned line going up the tower, approximately 125' from the antenna feed point. One dipole leg plus the whole tuned line is close to a full wavelength on 160.  DC input to the transmitter was 75 watts, so rf output power is estimated to be approximately 50 watts.

1800 kc/s: with OWL, rf current reading was 3.05 A.  With original coax configuration, 2.62A   Load impedance calculates to 5.38Ω.  Power delivery ratio, OWL vs coax: 1.36:1

1900 kc/s: OWL, 2.75A.  Coax, 2.4A. Load impedance, 6.61Ω  Power ratio OWL vs coax, 1.31:1

2000 kc/s: OWL 2.45A.  Coax, 2.1A.  Load impedance, 8.32Ω. Power ratio OWL vs coax, 1.36:1


The load impedance for the short dipole calculated much lower than expected. I was expecting something between 10 and 20 ohms, since the rf ammeter is placed midway between a current and voltage loop. Efficiency of coax feed vs OWL feed averaged about 75%. Increased power loss  probably occurs in the transmitter coupling system to coax line, as well as loss in the coax itself. Previous tests into a dummy load indicated more power delivered to the 440Ω load with balanced L-network, than delivered to the 50Ω load feeding with coax and capacity tuning of the link.

In the 50's at my parents the Viking 1 loaded fine on 160 into a 80M dipole fed with 72 Ohm twin lead. Worked the commuter mobiles several days a week with no tuner plus loaded on 80 and 40 where I worked plenty of DX on 40 CW.

The 15M folded dipole was made the HB way with hollow phenolic dowels spaced 6" apart and fed with TV ribbon also loaded fine and I worked the world including WAC as a Novice with a HB 6AG7, 807 and later with the V I. Thats the last time I came close to anything resembling OWL.

OWL, old fashioned tanks, and tuners are overated  :P :o  Different strokes for different folks.......what works works to paraphrase Yogi Berra.

I wouldn't go that far--maybe if you have room for a bunch of antennas to cover HF and 160 you can do the coax feed thing for all of them.   I use it for the unbalanced ones, but if I didn't have ladder line and the Matchboxes I'd have a tough time covering 10 to 80 m. with two dipoles. 

  I am not sure how this applies to OWL specifically, but combining two equal gauge wires in parallel drops the effective wire gauge 3 units. The circular mils double, and the ohms are cut in two.

  So two # 16's is equal to #13,

  and two # 12's is equal to #9

  This comes in handy with insulated DC wiring, and for using bare magnet wire twisted tightly with a drill, and used to make a balun. I tend to use what I have, and this technique works well.

Jim
WD5JKO

The thermocouple rf ammeter I inserted in the feed line didn't lie, at least not for comparative readings.  ::)



Plus, it's hard to suspend a half-dozen separate dipoles off the top of a 120' tower.  I never did like the idea of cluttering the space over my house or shack with a jungle of dipoles, one for each band.  Not only does it look like crap, it's more stuff in the air to need maintenance, plus they all interact with one another when confined to the space available at most ham installations.



Jim, the #8 was for a coupling coil.  I wouldn't use two paralleled wires for that - a little too JS even for my liking .  I managed to acquire some #8 by dismantling an old air core inductor in which all the plastic support strips had disintegrated (a good example of why I never throw anything away).  It was already in the form of a coil, so all I had to do was enlarge it a little using a fabricated temporary form to re-form the spiral to the correct size.  I used bakelite strips and glued them to the wire with epoxy for the permanent retainers, and mounted the coupling coil over the main coil, suspended in place with the plastic goop squirted out of a hot glue gun.

For OWL, it would be better to  keep the wires separated and make up a 4-wire line in a rectangular configuration.  That would  give you lower Zo than with two wires each with twice the circular mills. Lower resistive loss due to the skin effect too, since 4 separate wires would expose more surface area for a given number of circular mills.

 

One decent trap antenna covers the same bands unless youre into the odd ones such as WARC and 60M. Use a pair at right sngles to compensate for directivity.
No tuner needed at or close to resonance. Much past 1 wavelength and all the lobes create nulls using OWL.


With a 120' tower Id have some rotating aluminum on top for at least 20 and up but Ive never heard you above 40M either. A couple or 3 trapped slopers covers 160-40 with switchable directivity and even gain and F/B if done right. Or just 2 trapped inverted vees at right angles to compensate for nulls on 80/40. If you dont move around much a tuner isnt even needed. A shorty 40M rotary dipole at 120' would be a killer or a shorty 2el even is very manageable.

That's a useful suggestion but my lot size doesn't allow for two low band dipoles at a right angle to each other--I have to admit only using traps a long time ago when I had a 4BTV for a brief time.   Traps violate an antenna rule of mine (which is certainly debatable) that prohibits anything outside that can dissipate power or crap out (always at night up in the air in January).  With ladder line what's outside is wire only which is about as basic as you can get--the matching device stays in the shack where I can get to it not up in the air.

Everything's got pros and cons--I'm not under the belief that open wire line has no downsides--Pin 1 Brown says noise pickup with coax is dramatically less than with open wire line for example (with dipoles, not verticals). 

I agree with KM1H that "upper band" use would be better with a Yagi.
My Cushcraft A3S is very unpredictable for reliability. Sometimes all bands work perfectly and another time only 40M works. I do not have $400 to find out what is going on up there.
I have been very happy with my new OWL re-design and the main dipole stretched out flat for 160M, 60 feet high. The 3KA Dentron tuner does a nice job.
I have about 160 feet of ladder line now.
I have walked the antenna with a sort-of field strength meter before and after the re-design and there is significant increase of RF out there now. Checking 160 and 80 M.

Im not against anybody using whatever they want whether it works good or not. What I dont like is when someone claims their wire is superior to anybody elses when its purely subjective.

My 75M inverted vee at 180' lays down a dynamite signal....usually.... but having both a low angle and NVIS lobe generally helps especially for DX at grayline when the band is changing and the EU and JA/VK can be high or low. Ive busted many a pileup against some serious big guns with it and other times Im left playing with myself :o I have a sloper starting at 100' on another tower to compare with.

If that's the case, something is wrong.  Sounds like common mode is leaking through somewhere.  OWL and coax should reject noise equally well if everything is properly balanced and any opportunity for common-mode antenna current along the feed line is suppressed.  The receiver should hear only the signal that appears right at the feed-point of the antenna; the type or length of feed line running from the antenna to the receiver should make no difference.

Mine measure about 10% difference in current on each side i.e if 100 ma. is on one side, 90 ma. is on the other.  That's probably the result of one end of the dipole being held up by a tree and the other end held by an aluminum mast, albeit one that's insulated and floating off ground.

I would suspect that very few could have a perfectly balanced OWL 24/7.

Just having it appear balanced because an rf ammeter in each leg of the OWL reads the same at one point of measurement, doesn't necessarily mean that the line is perfectly balanced.  Sometimes the standing waves on the two wires may be shifted apart just in the right manner that the currents are identical at one point, but different everywhere else.  For example, the point of measurement could be just prior to a current loop in one wire, and just past the current loop on the other, each reading slightly less than the current right at the loop.  Everywhere else before and beyond that point, the currents would be different up to the next spot where the pattern repeats itself. In such a case, the common mode current would be slightly shifted in phase relative to the differential current, creating a different interference pattern on each of the two wires.

Here's the latest results of the tests.

This time, feeding the 160m series-fed 1/4λ vertical monopole through a matching network with 440Ω OWL vs. 50Ω coax.

Measured impedance of the antenna feed point at the output terminal of the matching network at the Dawg House:

1800 kc/s  140Ω + j269
1900         225Ω + j363
2000         420Ω + j475

Feeding with 50Ω coax, using a simple L-network.  With fixed inductance and variable capacitor, perfect match SWR=1:1 zero reflected power achieved at each frequency by adjusting the variable  capacitor.  RF ammeter at the antenna feed point reads with TX output power adjusted to 75 watts:

1800 kc/s  0.683 A
1900         0.535 A
2000         0.40 A

Feeding with 440Ω OWL, using parallel tuned circuit with antenna tapped down on main coil.  OWL fed directly to 14-turn coupling coil. No resonating capacitor. Closely coupled to 26-turn antenna coil parallel tuned with variable capaicitor, antenna tapped on at 17 turns up from grounded end of coil.

1800 kc/s  "reflected" reading (0-1 scale): 0.015     Antenna current @ feed point: 0.741 A
1900           (perfect match)                   0                                                     0.57 A
2000                                                    0.035                                               0.39 A

Apparently the OWL feed system is more efficient at lower end of the band, but at the top end the coax feed system is equal to or slightly better than the OWL feed system.

Next, a series variable capacitor was inserted in the antenna line to cancel out the +j reactance. This was compared to a 215Ω non-reactive dummy load at 1900 kc/s, and the results were consistent. Coil taps had to be moved to achieve match.

With the antenna and series capacitor, adjusted to resonance at 1800, the antenna current read 0.742 amps.  Reflected on 0-1 scale read 0.

Changing frequency to 1900, reflected indication brought to zero by adjusting the series capacitor. Parallel tuning capacitor in resonant circuit re-adjusted.   Antenna current 0.55 amps

At 2000 kc/s, the lowest the series tuning cap would bring the reflected reading was 0.3.  Antenna current reading, 0.3 amps.

The series capacitor had to be re-adjusted with frequency, along with the main tuning capacitor.  Efficiency was about the same at low-frequency end of the band as without the series capacitor, after coil taps were re-adjusted. At 1900 the efficiency was  slightly  lower after both capacitors were re-adjusted for maxumum antenna current and minimum reflected power.

At the high end of the band, efficiency was much lower regardless of capacitor settings or tap locations.

The tuning network works best tapping the antenna directly onto the coil, without a series resonating capacitor. Reflected power can be reduced to zero or near zero all the way across the band by adjusting only the main tuning capacitor.  But efficiency drops off at the high end of the band to about the  same as with coax/L-network feed, whereas efficiency is substantially better at the low end of the band.

Will probably use this in the final circuit, since it seems to be the best compromise.

Don,

Thanks for  providing all the data and results.  I'm surprised by two things:  1.  the high reactance.  some would not surprise me at all but hundreds of ohms--I guess the dipole and feedline to it inside the tower makes for a wild Z on the tower.   2.  it is paradoxical to me that the current would drop off at the high end of the band (although I don't know why I think that), but since the Z goes way up I guess for a fixed power level to the load, that will happen.  I'll bet even with less current the antenna is more efficient up there near 2000 kc. 

It would be interesting to perform field strength measurements between the low and high ends of the band with a small amount of power, maybe 10 or 20 watts and a measuring instrument with a small vertical antenna a mile or two from your antenna.  Any receiver with a d'arsonval S meter and detailed scale on it would be good enough.  that would tell if the tower is more efficient at the high end of the band.

 

A 1% difference is substantial?

The DC input to the final is 100 watts.

Calculating power to the antenna, using the resistive component of the impedance and the formula I²R, yields:

76.8 watts at 1800

73.1 watts at 1900

63.8 watts at 2000

If those measurements were anywhere near close, it appears the system is very efficient at the low end of the band, but drops off steadily as the frequency is raised towards the high end.


With the coax feeding the L-network, current readings were:

1800  .683 amps   65.3 watts

1900  .535 amps   64.4 watts

2000  .4 amps      67.2 watts

Looks like the efficiency with the  coax/L-network is consistently in the mid 60s across the band, with a slight rise at the top end.


With this tuning configuration, the highly reactive load is fed to a tap on the antenna coil.  The variable capacitor that parallel-resonates the antenna coil is across the entire coil.  The coupling coil, which is bridged directly to the OWL is tightly coupled to the antenna  coil.  The resonating capacitor is adjusted until the reflected power meter between the OWL and the coupling coil reads minimum.  In this case, the tuning capacitor almost completely nulls out the reflected power at any frequency across the band, with no other adjustments.

This was surprising, since the tapped coil is being forced to transform reactance as well as resistance.  Placing a capacitor in series with the antenna to tune out the  reactance (which required entirely different coil tap settings) would work only across a small portion of the band.  At the top end, I was able to get the reflected power to zero, but the  current reading was only about 0.3 amps. That calculated to only about 38 watts!  The tapped-coil matching network functions more efficiently with the complex impedance than with a purely resistive load.  Using the series capacitor, to QSY across the band required adjustment of the resonating capacitor, series capacitor and moving coil taps.

Interestingly, the OWL beat out the coax in every test on every band, except with the 160m vertical at the top end of the band.

The weirdness doesn't surprise me a whole lot, since with the 80m dipole attached near the top of the tower, the antenna actually functions more like a vertical tee.  With the quarter-wave vertical, plus 1/8 wave legs of the dipole acting as a flat-top, that gives a total antenna length of 3/8λ... one of those odd eighth-wavelength situations midway between a voltage loop and a  current loop, which turn out highly reactive and usually difficult to match.

Also interesting is that the resistive component of the antenna impedance takes a big jump between 1900 and 2000, but the reactive component increases at each step at 1.8, 1.9 and 2.0 roughly 100 ohms.