[TowerTalk] RF current in REF + DIR ..how much ?

[Jim Thomson]

Date: Wed, 22 Dec 2010 07:20:57 -0800 (PST)
From: Dennis OConnor <ad4hk2004 at yahoo.com>
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 degree..
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 of Z...

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