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[TowerTalk] Common-mode current on feedline

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Subject: [TowerTalk] Common-mode current on feedline
From: (Chuck Counselman)
Date: Thu Jun 26 14:02:34 2003
Howard K2HK wrote:
>----Original Message Follows----
>...I was able to find one _fixed_ setting of the tuner for which the 
>SWR on the coax (and therefore the loss) was tolerably low across 
>_all_ HF bands 80 through 10 meters, except 30 meters where I don't 
>transmit much power anyway.  Since then I've left the tuner fixed. 
>Now QSYing is much faster....
>73 de Chuck, W1HIS
>This is becoming even more interesting !! I am looking forward to 
>seeing your method. I wonder if it would apply to a shunt fed tower. 
>Substituting a fixed value for a variable capacitor for example. I 
>hope we hear some more of this.
>73, de Howard..K2HK

Here's an outline of my method.  There's not much to it.

First, my antenna is a doublet 30 meters (102 feet) long, like a G5RV.

My tuners have been C-L-C "T" networks; these are high-pass filters. 
I start by adjusting them for a perfect match on the lowest band (80 
m in my case).  Thus, they are relatively transparent on the higher 
bands.  The lowest-band match places two constraints on the three 
component (two C's and one L) values; one degree of freedom remains. 
(One does not have unlimited freedom here; tuner losses must be 

The length of the parallel-wire line between the antenna and the 
balun can be varied; this is another degree of freedom.  A change of 
just a few feet matters much more than you might think.  After 
changing this length you must re-adjust the tuner.

The length of coax between the balun and the tuner can be varied; 
this is a third degree of freedom.  Here again, you'd be surprised by 
how much a few-foot change matters.  Again, after changing this 
length you must re-adjust the tuner.

By varying these three free parameters you can achieve a perfect 
match on a second band (e.g., 40 m) and can minimize but not 
necessarily zero out the SWR on a third band (e.g., 20 m).

The first time I tried this method, I was lucky.  I got perfect 
matches at my target frequencies of 3.7 and 7.1 MHz (this much was 
not luck), and a high but tolerable SWR of 4:1 on 20 m.  Thirty 
meters was terrible but my transmitter, coax, etc., could handle it 
because I transmit only 200 W on this band.  The SWR was 2.0 or less 
across 17, 15, 12, and 10 m.  I could transmit legal-limit power on 
every band (1500 W, except 200 W on 30 m) but I burned out my balun 
(a commercial product rated at 5 kW) transmitting less than the legal 
limit on 20 m.

When I tried the method with a different tuner, a different 4:1 
balun, and a different common-mode choking arrangement, my luck was 
not as good.  I got perfect matches at my target frequencies of 3.7 
and (in this case) 14.1 MHz (again, this much was not luck), but a 
higher, less tolerable, 6:1 SWR on 40 m.  Thirty meters was still 
terrible; but my transmitter etc. could still handle it at the lower 
power limit.  The SWR was still 2.0 or less on 17, 15, 12, and 10 m. 
I could transmit legal-limit power all bands including 40 m (the 
tough one now) because my new balun (homebrew, and good for 30 kW 
with SWR = 1:1) was bigger than the old one, and my coax is Heliax 

Additional degrees of freedom are available but I have yet to exploit 
them because they require outdoor work in addition to turning knobs 
and changing coax jumpers indoors.  The length of my doublet (so far 
fixed at 30 m) and the characteristic impedance of my open-wire line 
are the leading candidates to be varied; but, in principle, there are 
infinitely many more, e.g., the ratio of the balun transformer, and 
all sorts of transmission-line stubs and matching sections.

If I had accurate measurements of the feedpoint impedance of my 
doublet antenna at all relevant frequencies, then I would let a 
computer program do the simultaneous multi-variable optimization. 
Microsoft Excel could probably do it.  The relevant transmission-line 
and lumped-element-network formulas are not too complicated, and I 
have Excel macros for most of them already.  Unfortunately, so far 
all of my several attempts at antenna impedance measurement have 
failed.  Strong signals from nearby 10- and 50-kW AM broadcast 
stations have clobbered the detectors of the simple impedance bridges 
that I've tried using, and the noise bridge that I've tried had 
insufficient range and precision.  I tried the conjugate-matching 
trick with a couple of antenna-tuners, but the accuracy I got seemed 

Right now I see this antenna impedance measurement problem as the 
Gordian knot, the key to achieving the Holy Grail of an all-band, 
no-tune, full-power, efficient, antenna-feedline system.  (Forgive my 
metaphors; I'm frustrated.)

73 de Chuck, W1HIS

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