On Sun, Sep 25, 2016 at 12:25 PM, Jim Brown <jim@audiosystemsgroup.com>
wrote:
> On Sun,9/25/2016 6:49 AM, Steve London wrote:
>
> What problem(s) are we attempting to solve with chokes on the 4-square
>> feedlines ?
>>
>> Noise.
>
Noise, yes. Gotta give you that one, but the main reason for the 4 square
was TX gain, right? You do have listening antennas for 160 and 80? So we
have to worry about *loss*. Loss eats up gain from patterns. Loss eats up
amplifier output. Your *system* gain past your transceiver is antenna gain
+ amplifier gain *** minus LOSSES ***.
Diversion of power to miscellaneous conductor paths is almost universally
lossy and never in directions and modes desired in our attempts at
directional arrays.
The common mode path on the outside of coax shields is lossy first because
it has a thick jacket which is never designed for low loss, second because
it often lays on the ground or is buried, a very lossy situation. Try your
20 meter dipole laying on the ground to check that out if you're not
convinced. And if that is not enough, coax is often taped to towers, making
it a transformer winding to the metal of the tower, whose reradiation is
certainly not going to be useful in a 4 square.
In correspondence about FCP conversion projects, I always advise installers
to ALSO go after all those death by a thousand paper cuts losses, because
the FCP installers are physically into a lot of the situations where corner
cutting costs transmitted power.
The first 0.3 dB loss to your brand new 1500 watts out takes away 100
watts. The second 0.3 dB takes away 93 watts, the third 0.3 dB takes away
87 watts. A 1 dB loss drops you to 1192 watts.
So three paper-cut 0.3 dB losses effectively drop your 1500 watt Alpha 9500
to 1200 watts. If your 1500 watt amp would only put out 1200 when you got
it from the factory, you would send it back to the factory, right? No? 0.3
dB losses can't be significant.
"Oh, it's all right, it's only a dB, that's fine, I love my new amp. 1200
watts is the same as 1500 watts." <smile, smile, warm smile>.
Actually, upon careful reflection, what I think I really heard was
!@%&*@!!!!! #$@%@%!!!!! #^#$%^!!!!!. <pant> 1200 watts!!!!! <pant,
pant> Do they think I'm an idiot ???? <pant, pant, pant> <Finger nails
on chalk board type scream>.
Oh, that's better. For a moment I thought you were overdosed on Elavil.
If you see a preventable 0.3 dB loss, step on it like a roach and kill it
before it multiplies. Don't put up with any 0.3 dB loss you don't have to,
and if you do have to put up with it, know *exactly why* you have to. Then
that 4 dB you can count on actually gettting out of a 4 square for sure,
regardless, will really mean something. I have a list of nearly thirty
different ways to waste 0.3 dB of RF energy. When I get done it might be
more than thirty.
Common mode losses can be way more than 0.3 dB.
Remember.... Death by a thousand paper cuts.....
I won't go into why and when isolation transformers can be be better than
ferrite based balun and choke type windings. That's a whole other story.
I have a design for an upside down 80m 4 square for mounting around a
tower, that does not touch the ground, does not involve large RF fields in
or near the ground, and uses the top Philly-stran guys to support FCP's as
counterpoises for the upside down verticals, and deals with common mode in
a way that is definitely out of the traditional "you gotta have 1/4 wave
radials" box. But it's not ready for general publishing. Anyone interested
in trying this experimental design, email me off list.
73, and good luck on the 4 square.
Guy K2AV
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