[TenTec] Built in SWR meter bannans

Wed Apr 24 05:28:44 EDT 2013

Steve,

Thanks for walking me through the process.

The beams aren't really the problem, although most beams in amateur use
probably are higher than 2:1 on one end of each band.

My former problems were with attempting to match openwire fed dipoles on
certain bands.  As we know, we're looking at 2000+ Ohms here.
That is quite a challenge for any balun.  I've found choosing 2x 17m or 2x
13m dipole lengths and avoiding resonant feedlines on any band to be much
easier to match than a dipole with the common recommended 2x 20m.  And of
course when it's not easy to match, that's when the balun is also
challenged.

Recently I have switched focus from center fed to off-center-fed dipoles,
which has enabled me to get the impedance under 600 ohms on all targeted
bands.  With a 4:1 Guanella designed for 200 Ohms, that's still a 3:1 SWR on
some bands.  I've built about 10 baluns in the past 6 months, using your
recommendations as guidelines.

Using a two-core Guanella I can tame this antenna, but I have been trying to
get an ultra lightweight version working, using a single 1.4" core and have
not managed to get enough CMC suppression.  I can use a different feedpoint
split and reduce CMC considerably, and get away with just one toroid, but I
lose one band and get higher SWR on a couple others.  I've even tried the so
called "New Improved Balun" by N7ZWY which is claimed to have better CMC
suppression, but my field tests have not substantiated that.

What I have not tried is the -31 mix.  However I have been testing on 40m
and above so there is probably not much advantage in switching from -43 to
-31; at least that's what I understood from studying the charts in Jim's
paper.  Maybe what I'm trying to achieve with my Ultra-light is impossible!
(hope not)

Thanks again Steve.  You are an excellent resource for our group!
Jim, likewise!

73
Rick, DJ0IP

-----Original Message-----
From: TenTec [mailto:tentec-bounces at contesting.com] On Behalf Of Steve Hunt
Sent: Wednesday, April 24, 2013 10:19 AM
To: Discussion of Ten-Tec Equipment
Subject: Re: [TenTec] Built in SWR meter bannans

Rick,

It's not difficult - just a bit tedious - to work this stuff out. Let me
walk you through the example of a feedpoint 1:1 balun for a 40m beam:

1) Assume the power level is 1kW and the beam feedpoint impedance is a
nominal 50 Ohms
2) Then we know the differential voltage across the feedpoint must be 224v
3) If we assume the beam is inherently well balanced wrt ground, the voltage
to ground from the coax at the feedpoint must be about 112v.

How much balun CM impedance do we need for adequate current balance?

4) Assuming again the antenna is well balanced, the impedance from the braid
side of the beam to ground will be 25 Ohms
5) Assume a worst case, that the CM impedance back along the coax braid is
very low
6) Then if we aim to keep the current balance in the beam to better than
-20dB, we need the choke CM resistance to be a minimum of 250 Ohms

*** Unfortunately that's where most folk stop - they ignore the choke
dissipation considerations ****

7) Say we choose a choke with a measured CM impedance of 4800-j1000 (typical
of 12 turns on an FT240-31) - clearly it meets the current balance
requirement easily
8) The current through the choke will be 23mA (112 / Zcm) and the power
dissipation will be 2.5W (0.023^2 * 4800)
9) I generally work on a cautious "rule of thumb" to limit dissipation in
one FT240 core to 10W - if you want to look at the detail, look at the
Fair-Rite thermal data. We are well within that limit at 2.5W!

However, re-visit some of the assumptions to see their impact:

10) If the beam SWR was not 1:1, but 2:1 at the band edges that could
increase the feedpoint voltage by SQRT(2) and increase the choke dissipation
from 2.5W to 5W
11) If the beam was not inherently well balanced wrt ground, that could also
increase the feedpoint voltage and increase the choke dissipation further

We're now beginning to approach the 10W limit, however because it's a
thermal limit we can de-rate by the duty cycle of the transmission mode. 
So I would be very happy to use this choke for 1kW SSB or CW in that beam
application.

As an aside, if we'd simply focussed on current balance and chosen a choke
with a CM resistance of 250 Ohms, re-working the maths it could dissipate
100W !!!

You might like to work through the same analysis for a 1:1 balun used at the
output of an unbalanced tuner looking into a worst-case ladderline impedance
of 4000+j0 when feeding a multiband doublet - finding a satisfactory balun
for that application is a much greater challenge :)

Hope some of that helps,
Steve G3TXQ

On 24/04/2013 08:18, Rick - DJ0IP / NJ0IP wrote:
> I would like to see some real life examples:
> How much do we need in a beam?
> How much do we need in a dipole?
> How much do we need if the choke is between an openwire fed dipole and 
> an asymmetrical matchbox?
>
> SPECIFICS please.  "Enough" is not very helpful.
>
>

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