Date: Fri, 29 Apr 2016 14:50:52 -0700
From: Jim Brown <jim@audiosystemsgroup.com>
To: towertalk@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
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