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Author Topic: Antenna Analyzer for Open Wire Feeders  (Read 40324 times)
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aa5wg
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« on: September 17, 2011, 05:56:42 PM »

Hi to all:
Is there an antenna analyzer that is designed for open wire feeders, i.e 300 ohm, 450 ohm, 600 ohm etc., that is accurate?
Chuck
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KA3ZLR
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« Reply #1 on: September 17, 2011, 06:34:51 PM »

Hi,

 Well I think a good place to start is what volume of signal are you planning and what type of equipment you plan on using Commercial. Homebrew give the group an idea of your station
set up and plans..Then go from there...


73
Jack
KA3ZLR
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aa5wg
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« Reply #2 on: September 17, 2011, 07:43:25 PM »

Hi Jack:
At present I am using a center fed 75 meter doublet test antenna.  There will be different feed-lines ranging from 300 ohms to 600 ohms and possibly a little higher.  The rig in use for testing will be a FT-101FX or a Drake C-Line or a Drake TR6.  The power range 1-120 watts 160 - 6 meters.
Chuck

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KA3ZLR
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« Reply #3 on: September 18, 2011, 07:03:15 AM »

Hi again,

 Here I found a good list of Analyzers I want you to know they can get quit expensive it's on eham.

http://www.eham.net/reviews/products/31

Here's an antenna coupler for Balanced lines alot of us built and use you can design it for your power situation:

http://amfone.net/ECSound/K1JJ13.htm

Here's a Neat little device you can Build yourself monitor your feedlines man:
(Click on Antenna Current Meter)

http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=25435

Hope this Helps a little bit O.M. Smiley
And remember building is more fun than Buying.. Grin

73
Jack
KA3ZLR


 
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flintstone mop
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« Reply #4 on: September 18, 2011, 09:38:39 AM »

There does not seem to be an analyzer for open ladder line. The ones listed in eHam are for 50 ohms.

The best way is pretty simple with just some small indicator lamps with their wires coiled on the actual ladder line and you tune for max brightness. If there is unequal brightness then you change the taps on the tuner components to have equal brightness.
The rigs you are using are unbalanced 50 ohm output. You're kinda stuck in using a tuner to transform the ladder line or open wire feeders to 50 ohms to make the radios happy.
Fred

This was nice info from eHam:
http://www.eham.net/articles/9751
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Fred KC4MOP
KA3ZLR
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« Reply #5 on: September 18, 2011, 09:49:36 AM »

I give up,

Mop you don't read much yea that's what I found looking quickly it's a good bunch to look at
but of No good use...Then......................hard to figure that..

I listed a Coupler that he can build and use very effectively and a Metering System he can build and use effectively he doesn't need an Analyzer...Just an Effective Coupler and a Way to monitor his Feed lines..

Just step on over me man......

Cheeze..........the gift of my post was Building what is needed...it's right there in front of you

Damn..

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KM1H
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« Reply #6 on: September 18, 2011, 11:02:46 AM »

If you know the true line impedance then you can wind a binocular core transformer to match the 259B or other analyzers.

I did that for my 2 wire Beverages using unknown cables, measured wire size and spacing, and went to an online calculator for the impedance. Use a carbon pot on the transformer to confirm a match where its supposed to be.

Carl
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k4kyv
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Don
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« Reply #7 on: September 18, 2011, 11:35:50 AM »


The best way is pretty simple with just some small indicator lamps with their wires coiled on the actual ladder line and you tune for max brightness. If there is unequal brightness then you change the taps on the tuner components to have equal brightness.

Or better still, thermocouple  rf ammeters, preferably two each, one in series  with each feeder.

But shifting taps on the tuner coil to achieve equal brightness does not guarantee balance in the feeders.  The currents may be equal at the point of measurement, but what about 1/8λ or 1/4λ down the line? The standing wave patterns on the two feeders may be significantly different, and adjusting for equal currents at one isolated point would do nothing but give a false sense of "balance".

Assuming the OWL is working into a  reasonably symmetrical load, unbalance in the feeders is most likely the result of common mode "antenna" currents along the feed line.  Each feeder is biased with an additional current, usually of the same amplitude and phase with an identical standing wave pattern, and it is the sum of the normal (differential) current and common mode current that is indicated by the meter. Grounding the tuner and futzing with taps on the coil is apt to make matters (common mode currents) worse.

The best way to avoid common mode currents is to use a balanced, link coupled tuner, with no direct ground connection anywhere to the main coil. The separate link coil isolates the main coil from ground, allowing it to float. An rf choke attached at the "cold" spot on the coil is OK for taking static charges to ground, without affecting the RF. Even with a floating coil, it is sometimes possible to still have common mode currents, particularly if the length of the feedline is  such that a current node (high rf voltage point) occurs right at the end - IOW, the transmission line may act like an ungrounded halfwave (or other even number of quarterwaves) "no radials necessary" vertical (including its inherent losses).

The only way to check for true balance is to use two sets of lamps or rf ammeters at two separate points along the line. Avoid placing these two points close to a half-wavelength apart at the operating frequency. Although unbalance is often, if not usually, a result of inadvertent, parasitic, common mode currents, it may also be the result of an inherently unbalanced antenna. An example of this would be the half-wave end-fed zepp. The "dead" feeder that is attached to nothing will not have exactly the same current as the active feeder, even right at the resonant frequency of the flat-top. As the transmit frequency is moved away from exact resonance, additional unbalance inherently occurs as the current node on the active feeder/flattop combination slides away from the exact point where the end of the flat-top attaches to the active feeder, moving to a point either up on the flat-top or down the feeder, while the current node on the dead feeder is fixed in position at the unterminated end of the wire, regardless.  This is an unavoidable trade-off with the end-fed zepp, and is usually not a significant problem if precautions are taken to avoid common mode currents via a grounded tuner coil, thus preventing the feeders from serving double duty as a (poor) grounded vertical  radiator.

From what I have seen, the most promising Antenna Analyzer for Open Wire Feeders is the Micro-match, as described in a late 40s QST. It is designed to measure SWR in a 50-300Ω balanced transmission line.  I believe that, with careful construction, it could be made to work up to 600Ω, at least on the lower frequency bands. My General Radio antenna impedance bridge is strictly designed to measure an unbalanced load with one side grounded.  I'm not sure if a commercial unit exists that is designed to accurately measure the impedance of a balanced load to ground.
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Don, K4KYV                                       AMI#5
Licensed since 1959 and not happy to be back on AM...    Never got off AM in the first place.

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KA3ZLR
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« Reply #8 on: September 18, 2011, 12:15:47 PM »

OK, Now that the Lecture is over did we read his Requirements  anyone..?

A 75 meter Doublet capable of operating 160-6 metrs with a potential
of 100-150 watts of Power and there will be changes made in the feedlines.

Rather simple I'd say I put up there JJ's coupler and a feed line metering system
capable of handling his power requirements...that he can build relatively inexpensively
and  he doesn't need thermal couplers  they'll stall out his signal before it gets to the
friggin radiating elements sheezee... he's operating QRP NOT QRO CCS.

Unbelievable...isn't it better to try an suggest inexpensive ideas...

Best 73 bud...good luck on your system..
Jack
KA3ZLR
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w3jn
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« Reply #9 on: September 18, 2011, 12:44:57 PM »

Chcuk, since you're obviously going to feed it with coax from the xmitter into a tooner of some sort, there's really no reason for a balanced line antenna analyzer.  Not sure what you'd want it to tell you.

Treat everything upwind of the xmitter as a system, that you want to appear as a 50 ohm resistive match to the xmitter.  Just affix a MFJ or whatever analyzer to the coax, go to town with the tuner, mark your settings, and be done with it.
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KA3ZLR
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« Reply #10 on: September 18, 2011, 01:22:11 PM »

Eggxactly.. what do we need with the analyzer.....My thoughts also, I think he should build the "SupaTuna" as Described and save money and build the Antenna Current Meter for practically
nothing , all he needs Quote JN "Upwind" respectfully is a VSWR meter...Simple play with all the
line inputs ya want...Keep yer money in your Pocket....

73
Jack
KA3ZLR

P.S. And hey if he really needs to see what's what's Get a decent Dip meter
       they're cheaper and werk just as well...

http://www.ebay.com/itm/EICO-MODEL-710-GRID-DIP-METER-/300599725452?pt=LH_DefaultDomain_0&hash=item45fd23cd8c
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ke7trp
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« Reply #11 on: September 18, 2011, 03:41:01 PM »

Clearly the Grid dip meter would be the best way to me. I have a book by sams on how to use a grid dip meter and one of the sections is many ways to use the meter when building and tuning an Open wire line antenna.  Maybe I can photo copy the pages for you.

As for balance. You can use Xmas Tree bulbs. Not the small ones. But the larger type. Please look up kc6mcw on QRZ.com and see his schematics. 

I use a cheap MFJ balanced line meter.  It shows current up to 3 amps per leg on two meters.  This was instrumental for accurate adjustment of the balanced tuner. Not only for balance but for effeciency.  Mistuning the link coupled tuner can make the transmitter happy but the effiency way way down. The current meters will allow you to tune for the highest output current while keeping a good match to the transmitter.  Simply swapping the line left to right can sometimes fix a huge imbalance. Without the meters, you wont see this.

The last tool that nobody mentioned is a field strength meter. This is a must have to me. You can take baseline readings as you walk around the line,   When balanced, the radiation at the lines will be much lower.  Later, You can leave the meter on a shelf or table and when mistuning, you can see the IN SHack RF come way up as the line in the shack or the tuner starts radiating.

Good luck

Clark
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Walt, at 90, Now 92 and licensed 78 years


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« Reply #12 on: September 18, 2011, 04:11:06 PM »

Chuck, what isn't clear to me is what type of tests are you  contemplating?  What are you looking for in changing the feed lines? Simply the 'balance' of current flowing in the two wires? What?

Seems to me we need to know your test goal to determine what test equipment you need to perform it.

Or has my aged mind missed something already stated?

Walt
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aa5wg
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« Reply #13 on: September 18, 2011, 05:35:08 PM »

Jack, Fred, Carl, Don, W3JN, Clark, Walt and all:
I just got back from a cerebration for my Uncle.  Sorry for the delay in replying.

Lots of good information everyone provided.  I will be using a home brew link antenna coupler with series and parallel ability.  I will be able to select series with two output capacitors or series with two output inductors with the line connected in the center of these two coils.  (This is the tank coil mechanically cut in two parts with the line connected between the two coils and in series with the tank capacitor)  Traditional parallel tune will be the third option.

When the entire antenna system length is midway between a current loop and a voltage loop, at the coupler and line connection, I would like to find the reactance required to bring this antenna system to resonance.  I would like to try series inductance (both sides of line) or series capacitance (both sides of line) or shunt inductance or shunt capacitance, if needed, on the line between the coupler and ladder line.  Then, I would like to compare the reactance requirements for this midway point for different line types, i.e. 300 ohm, 450 ohm and 600 ohm.

Today, I would like to be able to calculate the predicted reactance (inductive or capacitive) required to bring the above antenna system to reaonance and compare this calculation to an accurate measurement of actual required reactive components (inductive or capacitive) needed to bring the entire antenna system length to resonance.  

No Walt, I was not clear in stating my goal.

I am not the best writer so if I am not clear please let me know.
Chuck
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k4kyv
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Don
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« Reply #14 on: September 18, 2011, 07:43:27 PM »


When the entire antenna system length is midway between a current loop and a voltage loop, at the coupler and line connection, I would like to find the reactance required to bring this antenna system to resonance.  I would like to try series inductance (both sides of line) or series capacitance (both sides of line) or shunt inductance or shunt capacitance, if needed, on the line between the coupler and ladder line.  Then, I would like to compare the reactance requirements for this midway point for different line types, i.e. 300 ohm, 450 ohm and 600 ohm.

I had  that exact same problem shortly after I moved back here when I first put up my present antenna system, before I had installed  the ground radials for the 160m vertical. In order  to get on the band, I decided to load the 80m dipole up as a quarter-wave dipole on 160. (I didn't waste my time trying out the quarter-wave vertical with no radial system). Problem was, the feedpoint of the OWL was just that - midway between a current loop and a voltage loop.  I threw together a balanced link coupled tuner, using the split-stator plate tuning capacitor out of a BC-610 , and tried tapping down on the coil. I found a spot that gave a perfect 1:1 SWR looking into the tuner, but I couldn't modulate more than about 100 watts 100% before the 7 KV BC-610 capacitor would arc over. That tuner simply wasn't capable of handling such a highly reactive load very efficiently. Instead of futzing around with series or shunt capacitors and/or inductors to cancel out the reactance, I simply added an additional 1/8λ of OWL that switches in automatically whenever I select that tuner, which is parallel feed using a 300/300 pf 7 KV split stator bread slicer, with an additional 50 pf fixed vacuum cap shunted across the whole thing. It loaded (loads - I still use it) up perfectly, and I could modulate a KW DC input 150% positive, with no sign of arc over. But it is extremely sharp tuning - I can QSY maybe 5 kc/s before having to re-resonate the tuner coil. But most of the time when I operate 160 now, I use the vertical.

OK, Now that the Lecture is over did we read his Requirements  anyone..?

A 75 meter Doublet capable of operating 160-6 metrs with a potential
of 100-150 watts of Power and there will be changes made in the feedlines.

Rather simple I'd say I put up there JJ's coupler and a feed line metering system
capable of handling his power requirements...that he can build relatively inexpensively
and  he doesn't need thermal couplers  they'll stall out his signal before it gets to the
friggin radiating elements sheezee... he's operating QRP NOT QRO CCS.

Unbelievable...isn't it better to try an suggest inexpensive ideas...  

Look at the title of the thread to see to see what he was enquiring about. I didn't see any mention that the analyser had to be "inexpensive"; he wanted something accurate. Accurate and inexpensive are not necessarily the same thing.

And, thermocouple rf ammeters will not "stall out" the signal.  A good one will have negligible voltage drop and negligible insertion loss.

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aa5wg
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« Reply #15 on: September 18, 2011, 09:27:17 PM »

Hi Don:
Glad you got your antenna working on 160 with the extra feed line.   For me, I want to try capacitance (variable capacitor) or inductance (tapped coil) to cancel the high reactance midpoint between current loop and voltage loop.

I wonder how you determine if the line is capacitive or inductive reactive at this mid-point?

i.e. An antenna system total length is 150 feet long.  

246/frequency x velocity factor = one electrical quarter wave.  

i.e. 21.0 mHz = 246/21 x .95 = 11.13 feet for one electrical quarter wavelength.  Then, 150'/11.13 feet = equals 13.48 quarter wavelengths and a     mid-point between current loop and voltage loop.  

Is the above 150 foot long antenna system, at 21.0 mHz, capacitive or inductive reactive?
How reactive is it measured in ohms?

Chuck
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aa5wg
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« Reply #16 on: September 20, 2011, 10:34:03 AM »

Are there any more suggestions?
Chuck
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flintstone mop
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« Reply #17 on: September 20, 2011, 11:17:57 AM »

I'll take one more shot..........
IF your total length is 150 feet and you are using 6 inch spaced ladder line, you would be good down to 75M. Building the famous K1JJ tuner would be all that you need.
I think you might be complicating things worrying about capacitive and inductive reactance on the actual system. The tuner will make your radio happy. The rest takes care of itself. Get it up a minimum of 40 feet and it will serve you well.

This link will get you started..... by looking in the various pages you will see pictures and hopefully schematic drawings of this very nice tuner.

http://amfone.net/Amforum/index.php?topic=18054.0
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Fred KC4MOP
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"Season's Greetings" looks okay to me...


« Reply #18 on: September 20, 2011, 11:24:24 AM »

If I may add a few comments... in the spirit of trying to be helpful (even if some of the readers of this thread don't consider my comments helpful to them)

I believe that placing lumped circuit elements (capacitors or inductors) at the midpoint of a feedline that is longer than 1/8 wavelength at the operating frequency will be of little or no benefit in improving the performance of the feedline. The standing wave ratio (the ratio of maximum to minimum voltage and the ratio of maximum to minimum current) in the uncut sections of the feedline leading from the antenna to the midpoint of the feedline will be essentially the same. Therefore the loss (if significant) in that section of feedline and the peak wire-to-wire voltage in that section of feedline will be the same as it would be if you just used a tuner at one end.

It would be true that at certain frequencies, the wire-to-wire voltage would be less at the midpoint itself... and the effect of using lumped components at that point could be to reduce the required wire-to-wire voltage rating of the components in the tuner at the far end. But, as Don pointed out, one could achieve that same result by making the balanced feedline a little longer or shorter at that frequency.


In the telephone industry, years ago, loading coils were added periodically along long telephone lines to achieve an improvement in transmission performance... but the frequencies of operation were audio frequencies... and therefore the spacing between the loading coils was very small compered to 1/8 wavelength.

Stu
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aa5wg
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« Reply #19 on: September 20, 2011, 11:47:11 AM »

MOP and Stu:
Thank you for you inputs.  I agree, the tuner you mention is very good.  I have built it and it works well.  By trial and error with the taps and input and tank output capacitor setting changes I am able to load wide sections of the bands.  But, I would be like to have the ability to predict and measure the actual reactances required to bring the total length of the antenna system to resonance.  That is why I was looking for an antenna analyzer for open wire lines.

If you have time look at my earlier comments in this thread.  I was wondering how I would determine if the line, at the tuner, would be capacitive or inductive reactive and how much in ohms (calculated and measured)?

Chuck
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k4kyv
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Don
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« Reply #20 on: September 20, 2011, 02:01:27 PM »

Quote
For me, I want to try capacitance (variable capacitor) or inductance (tapped coil) to cancel the high reactance midpoint between current loop and voltage loop.

I wonder how you determine if the line is capacitive or inductive reactive at this mid-point?

A good question, and there must be a simple obvious answer, but it's something I'll have to think about, whether the reactance at the mid point between a voltage loop and a current loop is capacitive or inductive.

If the feedline terminates right exactly at a current loop, the load it presents to the tuner will be a low impedance and purely resistive. If it terminates exactly at a voltage loop, it will be a high impedance, and purely resistive.  The exact resistance will depend on the antenna being fed and the surge impedance Zo of the line.  If the line is slightly too long, but near a voltage or current loop, the tuner will see something resistive plus some inductive reactance, and  conversely, if it is slightly too short, it will be resistive plus some capacitive reactance. But if it is midway between the two, will it see an inductive or a capacitive reactance?

If you take a measurement with an impedance bridge, it will show up as some specific value of R ± jX (resistance in ohms ± some value of reactance in ohms), (+) indicating inductive reactance and (-) indicating capacitive. So if it is 1/8λ longer than what would terminate at a current loop but 1/8λ shorter than what would terminate at a voltage loop, would it be some value of resistance plus jX ohms, or some value of resistance minus jX ohms? If midway between the two, the resistive component would vary along a sine curve to some intermediate value between the adjacent high and low values, but off the top of my head I can't say if the reactance is plus or minus. Something I had never really thought about until now.
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aa5wg
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« Reply #21 on: September 20, 2011, 02:52:54 PM »

Hi Don:
This is very interesting to me.  I am looking at a graph on page 79 in the ARRL Antenna handbook, 1960, chapter on Transmission Lines.  The graph information states, "Reactance at the input terminals as a function of line length in wavelengths open-circuited line (there also is a graph for closed-circuit line).

Anyone please jump in here and let me know if I am thinking correctly.  
(1) Input terminals I assume is the connection point of the line and antenna coupler.  Yes?
(2) Open-circuited line I assume is like a dipole and not a closed loop.  Yes?

The graph shows the first quarter wavelength, 2nd, 3rd and 4th quarter wavelengths.  The zero point starts on the far left side of graph and goes to the right to the 1/4 wavelength mark (on the horizontal line of the graph).  Zero to the 1/4 wavelength mark is shown as capacitive reactive.  From the 1/4 wavelenght mark to the 1/2 wavelength point, going right on the graph, shows the reactance as inductive.

This graph is showing odd quarter wavelengths is the starting point of capacitve reactance and even quarter wavelength as the beginning of inductive reactance.  Thus, I think, (jump in here to correct me) my 13.48 quarter wavelength long 15 meter antenna system would be capacitve reactive.
I am guessing here.  13 quarter wavelengths is an odd number of quarter wavelengths.  The .48 length lets say is .5 or one half way between the current loop and voltage loop.  Another way to thick about this is .5 quarter wavelength could be a 45 degree point or a 1/8 wavelength.  

On page 80 there is a Universal reactance curve graph and two formulas.  I quote: "When a line section is used as a reatance, the amount of reactance obtained is determined by the characteristic impedance and the electrical length of the line.  In the case of a line having no losses, and to a close approximation when the losses are small, the INDUCTIVE REACTANCES of a short-circuited line less than a quarter wave in length is

XL (ohms) = Zo tan L

where L is the length of the line in electrical degrees ( i.e. 45 degrees) and Zo is the characteristic impedance of the line (i.e. 600 ohms).  The CAPACITIVE REACTANCE  of an open-circuited line less than a quarter wave in length is:

Xc (ohms) = Zo cot L

My section of line of interest is 45 degrees long (.5 quarter wavelength) represents 45 degrees.  Using the Xc = Zo cot L formula gives me this:

cot of 45 degrees = 1 x Zo (600 ohms) = 600 ohms capacitve reactive.  When I convert 600 ohms to pf of capacitance at 21 Mhz I get 12.6pf of capacitance.

Do you guys think this is correct, 600 ohms of reactance represents the mid-way point between current loop and voltage loop?  For 450 ohm line this would give us 450 ohms capacitive reactive and 300 ohm line would be 300 ohms capacitive reactive.  

Link antenna couplers seem to have their greatest challenges at mid-way points between current loops and voltage loops.  Series tuning would used when a current loop is at the coupler and line connection point and parallel tuning would be use if a voltage loop was at the coupler and line connectoin point.  But what about these mid-way points at the coupler line connection points?

To counter this 12.6pf worth of capacitve reactance, at 600 ohms, would require and inductive reactance of 600 ohms or 4.5uh.

What would be the best way to add this 4.5uH of inductance to the link coupler when we are at a mid-way point between current loop and voltage loop at the coupler and line connection point?   I have three circuit choice, (1) series tuning with two output capacitors, (2) series tuning with the tank coil split in two parts with each split in series with one side of the line or (3) traditional parallel tuning.

Could I shunt 4.5uH of inductance at the junction of the output tank capacitor and line using parallel tuning?

Could I add 2.25uH of inductance in series with the two output capacitors in series tune?

To me, using series tune with the tank coil cut in two parts would not be the way to go?  Adding 2.25uH of inductance in series with each leg of the two output inductors seems counter productive?

Please jump in here and let me know what you think so far?
Chuck





 

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ke7trp
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« Reply #22 on: September 20, 2011, 05:20:57 PM »

Chuck,  Is your goal?     What are you trying to achieve?   More bandwidth? Effieciency?


c
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« Reply #23 on: September 20, 2011, 05:32:38 PM »

c
I would like to be able to know the reactance at the coupler and line juction when the total antenna system length is at a mid-way point between a current loop and voltage loop at this junction.  I can then add the correct amount of reactance (shunt or series) to resonant the antenna system.

Chuck
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Are FETs supposed to glow like that?


« Reply #24 on: September 20, 2011, 06:04:57 PM »

The cheap little Autek analysers let you set the impedance.
Not locked into 50ohm measurements.

They work at LEAST as well as MFJ...

Except no analog meters.

http://www.autekresearch.com/va1.htm
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