Date: Wed, 22 Dec 2010 07:20:57 -0800 (PST)
From: Dennis OConnor <firstname.lastname@example.org>
Subject: [TowerTalk] RF current in REF + DIR ..how much ?
Jim, I would opt for the clamp on RF transformer...
Now, having said that, I recognize that anything we do to measure the RF
current is going to upset balance, impedance, etc. to a greater or lesser
I suspect the old RF meter ( I have a couple) is going to introduce enough
resistance in the element to upset the coupling between the elements... These
were originally intended to be used on open wire line at several hundred ohms
With the clamp on RF transformer you can calibrate your RF voltage relative
reading by using a signal source and either RF voltmeter or oscilloscope, or
both, to give you voltage readings that can be mapped out as amperes...
But the issue is what do you expect to learn? I don't see the ratio of
voltage/currents without being able to measure vector/phase as being
informative... An example...
Two loaded short elements coupled to form a Yagi/Uda beam will have higher
currents in both elements than a full size, unloaded Y/U beam for a given
identical drive power...
A Y/U beam made of a folded dipole (300 ohm twinlead) driven element (or quagi)
and a shortened, loaded reflector will have significantly different circulating
voltage/currents between the DE/REF, yet still form a beam pattern...
This is a fascinating question that I don't have many answers for... Maybe
some of the gurus with electronic engineering background will discuss this...
I await with great interest...
denny / k8do
### Thanks for the info... and also to the other replies, from Kevin, etc, and
also from e-mail.
## The original SWR curves for my F-12 340N [3-el 40m yagi] were done by
N6BT,with the swr meter in his FT-1000MP.... with 400 feet of LMR-400 coax.
[1.32 db loss in coax].
That much loss in the coax is of course going to obscure the swr results. The
top of tower SWR results will be worse VS the SWR result 400' downstream.
I can't see the FB and Gain
holding up across the 300 khz either. I came up with the idea of using a
pair of aprx .56 uh coils and a DPST relay in a nema box to shift the
resonance point lower, esp the DE.
IE: the stock ant is 1:1 swr @ 7070 khz... and rises really fast on the lower
side..and slowly on the high side. The idea was to shorten the solid 3/16"
AL LL rods a bit.. and shift the
resonance higher... like 7150-7200 kz..... then use a .56 uh coil in each leg
at the feedpoint to lower the resonance back down... like down to 7025-7050
khz. On paper, this will work.
The 40m yagi will be at the bottom of the mast..and just above the top of the
tower. The DE is easy to get to since it's adjacent to the top of tower. OK,
then I thought, why not build up
2 x more box's..one for the REF and DIR. Same deal, the LL rods would have
to be shortened on the parasitic els as well. Each coil is made from 3/8"
silver plated tubing. 5-7 turns,
with a 1.5" ID. Next up is the relay. This time around, I want to use a vac
relay[s]. I only have so much room inside a nema box, so either 2 x spst
relays could be used to shunt the coil [one per coil]
in either a NC or NO config... or a single DPDT vac relay could be used.
## I looked at gigavac G-2's G-23's and G-20's and also G-12's, G-41A's,
G-43A and G-47A, G-8 and G-2. I have some of these on my shelves.
The G-12 is a DPDT, but the contacts are only good for 4-5A @ 7 mhz. I was
thinking of using one each, for the REF + DIR. Since we are talking
about YC-156 power levels, I had no clue what kind of RF current is in the
REF/DIR. It appears that from the various info I now have, current in the
parasitic els can easily be 60-70% of the DE.
That little G-12 is less than marginal..and really is not suitable.....esp
when they are out at each end of the boom = pita. The plan was to use parallel
control cable conductor's for
redundancy. IE: 4 x conductor's for each 12 vdc relay coil.
## It just occurred to me that a pair of either SPST-NO or a pair of SPDT
vac relays might be a better solution. Then I have the option of
[A] no coils, [B] one coil [C] both coils. Done this way, then I can
get 3 x resonance points..instead of just 2. The 340N really needs to have
it's stock 7070 resonance point
shifted down to 7025 khz. Stock, the swr skyrockets below 7070. Ok, then the
problem is the next resonance point. If it's too high in the band... like
7225-7250.... everything goes to hell
just below 7225. If the lower 7025 is used, then 7225 is now too high, you
just can't win with only 2 x resonance points. With 3 x resonance
points..then you can have ur cake and eat it too.
Like 7025-7125-7225 or 7025-7137-7250
## at least I now have something to work on, and will try some test configs.
later.... Jim VE7RF
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