Were We Talking About 160M Antennas?

[W9GT]

Well..maybe not within the last few days, but this seems to be a recurring topic of discussion.  We were talking about an idea last nite on 160 and wondered if anyone has tried it.

To resolve the problem of a narrow bandwidth antenna (loaded dipole) and accomplish reasonable performance and SWR over the lower range (1885) and upper range (1980-1995) of the band, I have considered a relay switching arrangement.  This would consist of a 3 ft (or so) extension on the ends of the loaded dipole switched in or out by utilizing a small latching-type vacuum relay on each end.  The relays could be switched by running a small DC control voltage thru the feed line or via separate control leads (via RF chokes to block the RF from the control power supply).  The relays would be the latching type....so no need to have the control voltage on except when changing the position of the relays.

I guess the big concern that came up in our discussion was the high voltage that would be present at the ends of an electrical half-wave antenna....hence the need for high voltage or vacuum type relays.  I wonder what kind of voltage breakdown would be necessary.  The small vacuum latching relays are good for several KV.

Just wondering what you guys think....for the sake of discussion...before I actually try it.   

73,  Jack, W9GT

[W1UJR]

That's an interesting idea Jack, but I do wonder what would happen if you had the relay at a voltage point.
In theory your location would be safe, but should the voltage point be shifted, say by the antenna not complying with the ideal model, differing perhaps in height, etc. then you may have an issue with arcing. 

Is your antenna that bothersome you can not just tune it out with a transmatch?
The set up here, using balanced line, plays on 160-6 meters.
Of course with loading, and coax, that might be a real problem.

My transmatch set up is shown below.
I use a good old Johnson Matchbox on 75-10 meters, and a Heathkit "air balun" on 160, the output network of the TX has enough adjust of the L and C to make this work just fine. 

What's the set up there, OM?

-Bruce

[W9GT]

Hi Bruce,

Well this particular antenna is coax fed....so the tuner is really not the way I wanted to go.  The tuner will allow me to load the antenna over a wide range, however, it is just making it look like some sort of reasonable match to the transmitter....not really "tuning" the antenna.   

I also use a balanced tuner for my 75 m antenna(s) with balanced feedline.  That works great and will actually work on all bands, however, not too great on 160M.
Actually, my best 75 M antenna is a "three-wire dipole" fed with big 600 ohm feeders and a remote balanced tuner that sits right under the center of the antenna.  I wish I had room to do the same thing on 160M!

I think the relay scheme will work OK, however, as I mentioned, the breakdown voltage of the relays will need to be fairly high, since the point that I will be switching is at, or near the ends.  I am wondering if the small Jennings latching vacuum relays will work.  I guess I just don't want to destroy the relays in the process of testing the concept.  I may just use some cheap open frame relays for testing first.

73,  Jack, W9GT

[K1JJ]

Jack,

The vacuum relay longevity will depend much on the pep power you run into the ant.... and the relay structure. of course.

I once used those 15kv small latching type vacuum relays to switch a coil in and out on my 75M quad array. I wanted to cover both phone and CW.  I had one on the driven el and the reflector near the base of each loop. They were bridged across a the coil to short it or open it.

They lasted about a day until they arced across the body of the relay and ruined it.  Bear in mind that they was a lot of voltage across the coil when opened.

Now your situation -  I would think the biggest threat will be the path into the relay coil from the end of the antenna. Be sure to run the relay control leads away at right angles from the antenna legs, of course. Go straight to the ground to minimize coupling.

It WILL work, but depends on the relay and QRO level. For example, I designed up a collinear array for Bob/W2ZM which consisits of two half waves in phase with a pair of refectors and directors for the 75M DX window. He has four vac relays - one each in the middle of each parasitic el to make it a dir or ref to switch directions. So far it works FB with no arcing. This will be similar to what you are doing, though his is at a current point, so big difference, actually.

The bottom line is if you ran 10 watts it would work FB. If you run 1500W, much will depend on the relay insulation structure. You might need the 35KV style vac relays for switching the ends like that. . Wish I had a more specific answer, OM.

73,
T 

[Steve - WB3HUZ]

The tuner will allow me to load the antenna over a wide range, however, it is just making it look like some sort of reasonable match to the transmitter....not really "tuning" the antenna.   

It IS tuning your antenna system my friend. If the antenna system (wire and coax) were not tuned to resonance, it would not take power.

[W1UJR]

The tuner will allow me to load the antenna over a wide range, however, it is just making it look like some sort of reasonable match to the transmitter....not really "tuning" the antenna.   

It IS tuning your antenna system my friend. If the antenna system (wire and coax) were not tuned to resonance, it would not take power.

The magic of the conjugate match, Kurt Sterba and Walt Maxwell all the way!
Ever read why the League and Maxwell had a falling out?
I understand that it was on that very point.

[W9GT]

The tuner will allow me to load the antenna over a wide range, however, it is just making it look like some sort of reasonable match to the transmitter....not really "tuning" the antenna.   

It IS tuning your antenna system my friend. If the antenna system (wire and coax) were not tuned to resonance, it would not take power.

The magic of the conjugate match, Kurt Sterba and Walt Maxwell all the way!
Ever read why the League and Maxwell had a falling out?
I understand that it was on that very point.

Well, I'm certainly not an expert theoretician, nor do I think that it is really useful or necessary to argue semantics.  I always thought that an antenna tuner was actually a device for matching the complex impedance of the antenna system at a particular frequency to the output circuit of the transmitter.  It doesn't "tune" the antenna.   Its tuning function is that of an (almost) infinitely variable transforming device for matching impedance. Efficiency can be very poor under some matching conditions or "tuning" control settings and you can end up dissipating power in the form of heat in the matching network components or in the transmitter itself.  Any way, just because you achieve apparent matching at the transmitter in the form of low indicated VSWR, doesn't mean that the power is actually radiating in an effective or efficient manner.

Now that being said.......I do understand that it is a challenge to get anything that is a reasonably good radiator functioning on 160M, just because of the physical size of a full-sized half wave antenna.  All kinds of schemes have been discussed in this forum that do work and get you on the band.  That doesn't mean that they are necessarily as good as a "full-sized" antenna, but they do work reasonably well.  My approach in this particular instance has been to experiment with a reduced size electrical half wave antenna through the use of  loading coils.  There is certainly some loss of efficiency, but it does work...and fairly well. The big drawback that I have experienced is the very narrow bandwidth and that is what I was hoping to resolve by switching an extra length in and out as required on the ends of the antenna.

I did not ever intend to represent myself as being an antenna or transmission line expert and that is why I appreciate the opportunity to discuss ideas in this forum to get some different opinions and insight.   

73,  Jack, W9GT

[W1UJR]

I did not ever intend to represent myself as being an antenna or transmission line expert and that is why I appreciate the opportunity to discuss ideas in this forum to get some different opinions and insight.   

73,  Jack, W9GT

Hey OM, no criticism intended, hopefully no offence taken.
Same here, I am no antenna expert, but have learned a great deal from Maxwell and Sturba's work, along with other here on the board.
And I'm still learning.

That power has to go somewhere, and for a while the League pulled Maxwell's work because they held the belief that the transmatch "fooled" the transmitter, rather than understanding that reflected power was actually radiated.

Walt actually has been good enough to put some of his book Reflections II on the web, see below:
http://www.w2du.com/Chapter07.pdf
http://www.w2du.com/r3ch19a.pdf


As aside, allow me to mention my clueless Johnny Novice years, cured only by asking questions, and helpful responses from others. I remember my Elmer used to tell me that the magic was in the antenna, something my young mind did not understand. I thought it was all about power, not the antenna!
When I think about where I started out as a young JN, Icom 765, Icom 2KL amp coupled with coax to a Cushcraft R-7 antenna on the roof of my house...gee, why am I not getting out? Why is RF present in my shack? Why is the band so noisy? Why will the antenna not load up? To the current situation, balanced feedline 160 meter dipole, balanced tuner (Johnson Matchbox), its like worlds apart.
Old technology never lets you down!

73 Bruce

[WA1GFZ]

Jack,
Look at this forum as a big design team and we all have different ideas. We dump them in the big stil and see what boils out. Even an idea that doesn't work is a good data point. We are all looking at the best configuration based on real information not hear say.

[W9GT]

I wasn't offended!!  I just think there was a momentary discontinuity or lack of concensus regarding the definition of an an antenna "tuner".  Probably only peripheral to the discussion at hand, anyway.   

I guess the main question that I was struggling with is the needed voltage rating of the relays for my proposed switching arrangement.  Assuming that I might run something resembling "full legal power" plus a little reasonable safety factor, I am wondering about frying some expensive relays if I don't get it right.

73,  Jack, W9GT

[WA1GFZ]

Tom's point was really high unless you run QRP. Not easy to do as he found when his fried.

[wa1knx]

I was planning something like that years back. separate power lines to switch the
relays I would think might fry. using two wires or coax for the element you
could push dc or ac down the feed for switching the relay.

Feed dc/ac through an rf choke from the coax shield on the center conductor side of the dipole to one wire. center conductor becomes the 2nd wire. put a cap across the two wires on that side after the choke.

Feed the dc/ac from the coax center conductor on the shield side of the
dipole through an rf choke to one wire. shield becomes the 2nd wire.  put a cap across the two wires on that side after the choke.

we now get rf right, and juice down the wires (or coax could be used) to switch.
there may not be all that much tension with only 3' being switched in. with
good insulators at end, might not be too bad

[W9GT]

We may have talked all around an obvious solution.  I received a nice email from Bob, WB9ECK who has had some experience with a similar antenna.  He switched in some series capacitance at the feed point (current point as opposed to voltage point of the antenna).  He was able to use small relays and it worked fine.  Seems like a good idea to me.   He also suggested some other arrangements with stub tuning , etc.  When it stops snowing, I may try some of the ideas!    

73,  Jack, W9GT

[Bacon, WA3WDR]

I got reasonable results with a two-turn square vertical loop 40 feet on a side on 160 meters.  I made a square loop out of 450 ohm twin lead (the kind with the cutouts and the 18-gauge copper-clad steel conductors), and fed it cross-connected at the ends, where they came together, like this:

---------------n/c     feed---------------
---------------feed     n/c---------------

40 feet on a side was not the self-resonant length for 160, so I had to tune it.   But it did work.  I first tried it horizontal, and I barely heard some other northeast AMers real weakly through unbelievable lightning static.  Then I wondered how they could be talking through that static, and I happened to put my hand on the tuning coil and they suddenly were audible.  I figured I was getting real low-angle static, and nulling the high-angle northeast AMers.  So I hung the loop sort of vertical - and not only did I hear them, but they heard me on it with my Viking II, so it couldn't have been too bad.

Some day I may see what length makes it self-resonant, and what impedance it presents to the feed, and what happens if you short the open ends, etc.

[Steve - WB3HUZ]

Bruce nailed it. Read Walt Maxwell's book, Reflections. Tons of good info on tuning, matching, feedlines and SWR. Lots of hammy muths are busted. And yes, Walt IS antenna and transmission line expert. I've read it three times and learn something new each time. Wish my pea brain could absorb it all!

[Bacon, WA3WDR]

I've noticed that when I have a lot of reflected power, my 80 watt output transmitter would produce 120 watts forward and 40 watts reflected.  That was because the transmitter put 80 watts into the system, the antenna got just about 80 watts from the system, and 40 watts was circulating around.

Antennas are about the same.  You put power in, a lot of it circulates in the antenna wire, and a little of that radiates.  Tuning the antenna synchronizes that circulating power, so that it builds up, and it builds way up until the moderate percentage of radiation leakage (plus loss) gets to be equal to the amount of power you are inserting.

You do whatever it takes to accomplish that tuning... and even if the tuner is at the far end of the transmission line from the antenna, you can call it tuning the antenna, because the transmission line is just part of the power recirculation system that builds up enough circulating power in the antenna for the antenna to radiate the entire input power, or at least the input power that is not absorbed by system losses.  In reasonable systems, the losses are relatively small, and most of the power radiates.

The recirculation has a Q.  This is the Q of the antenna system.  A typical dipole antenna has a Q of about 10 or 20, if I remember correctly.  Consider that to mean that a relatively small amount of the circulating current radiates.  Of course... the rest circulates around and comes to the radiator again, and some of it radiates along with some of the direct power from the transmitter.  And so on.

If the antenna is self-resonant, then it looks like a resistor at the feedpoint, and if the transmission line is the same impedance as that resistance, then power does not circulate in the transmission line, it only goes through and out to the antenna.  Power circulates in the antenna, but it presents a matched load to the transmission line in this example, so it doesn't go back into the line.  The crazy stuff going on in the antenna in this example simply acts like a resistance to the feedline.