[TowerTalk] chokes for antenna testing

[Jim Thomson]

Date: Fri, 29 Apr 2016 14:50:52 -0700
From: Jim Brown <jim at audiosystemsgroup.com>
To: towertalk at contesting.com
Subject: Re: [TowerTalk] chokes for antenna testing

On Fri,4/29/2016 7:35 AM, jimlux wrote:
>
> I think my real question has to do with "how much impedance is enough" 
> and "where should that impedance be placed"


In general, for a transmitting antenna, the first choke should always be 
at the feedpoint, or as close thereto as practical. Any length of 
feedline between the choke and the feedpoint is part of the antenna.  I 
recommend and use additional chokes along the line for two reasons. 
First, as "egg insulators" to break up the feedline just as we would a 
guy wire to prevent interaction with nearby antennas (primarily 
verticals). Second, to distribute some of the common mode voltage 
between multiple chokes to prevent overheating. VE7RF has suggested 
another reason to add a choke near the shack so that the feedline does 
not act as an RX antenna for equipment with RFI susceptibility, 
especially when that equipment has a Pin One Problem. Jim has a rack 
full of audio gear with Pin One Problems, and reports that a good choke 
at the shack end of the coax solves it.

>
> I'm not particularly concerned about RFI, for instance, but I am 
> concerned about coupling to the feedline and any asymmetry in the 
> system (and the antenna surroundings) perturbing the measurement.
>
> So, from that basis, I would think that you want chokes periodically 
> along the feedline, so that no piece of the feedline is "significant" 
> in terms of coupling to the antenna under test.
>
> And, for lower frequencies, the "significant length of a unchoked 
> piece of wire" in the near field is longer than for higher 
> frequencies.  A 5 meter long conductor near a dipole cut for 10 meters 
> is a big deal because it's a half wavelength, but probably 
> insignificant for 80 meters, where's 1/16th wavelength.
>
> So this would imply that "low frequency chokes" (e.g. 7 or 8 turns 
> through 5 cores) could be farther apart than "high frequency chokes" 
> (3-4 turns through 5 cores).

Agreed on all counts.

> Or, as Jim recommends for a 40-10 meter - 4 turns on 5 cores and 3 
> turns on 5 cores = this gets you >5k from 7 to 25MHz (fig 46 in the pdf)
>
> Is there an advantage in stacking cores other than ease of 
> building/mechanical?  I would think that 5 turns on 5 cores is about 
> the same as the series combination of five separate 5 turn on one core 
> chokes.

The reasons are both mechanical (getting enough turns of the cable 
you're using through the core to provide the needed inductance and 
capacitance) and for power handling. Inductance and coupled Rs is 
approximately proportional to the length of the cable within the core(s) 
and, of course, the square of the turns.


73, Jim K9YC

###  part of the issue is....how good is the ground at your SPG ?   Mine is just a 
large 1/4 inch thick AL plate, on the inside basement wall.   Then  5 ft of 2 ga  stranded wire from plate to
outside  8 ft ground  rod, cadwelded.   Then 40 ft of buried  bare  2 ga wire, from 1st grnd  rod to the base
of the tower, where its bonded to the  3 x 8 ft rods at base of tower.   Now does this mess make for a 
good  RF  ground at the SPG?    Probably not, but good enough for lightning.  Also ran 3 ga wire from 200A
main panel to the SPG plate.   3 years ago, my main cu water line  coming into the home, cracked, and replaced 
with  plastic PEX pipe.   So lost the original  bonding from cu pipe to 200A panel. 

##  Steve, G3TXQ  on his notes on CMC chokes..assumes a 20 ohm  SPG ground.  Well that could vary  from
A-Z, depending on each individual  SPG format.   This is where a CMC, sometimes  just  b4 the SPG..and also 
after the SPG, will work. In these cases, I assume folks are bonding the braid of their coax  to the SPG.   I also bond
braid to top of tower..and also bottom of tower.   Again, even with 3 x 8 ft grnd rods at base of tower, how good is that
for an RF ground at base of tower ?   Probably lousy.    You would not use  1-3 x  8 ft rods as a counterpoise on any 
ground mounted vertical. 

##  I can easily set off  my motion detector lights and also my neighbors, with just 400 watts.  I can turn on the gas furnace,
on 20m only, in fan only mode, with 1.2 kw.   CMCs, located in the correct places, on the coax of the feedlines,
can eliminate 99%  of these RFI issues.   Start there 1st,   vs  loads of  filtering on offending devices..which is a pita. 

Jim   VE7RF