I am getting comments from people who have installed FCP plus isolation
transformer 160 TX antennas, how their new TX antenna is quieter than their
K9AY or pennant, maybe a third or half of respondents (!!!) with some hint
or outright statement of this. But I think, rather than the TX antenna
being all that good, it's really the RX antenna's common mode isolation
really being that grotesquely BAD.
I can make this happen in a model by putting the coax on the ground and
making the now low velocity factor ELECTRICAL length of the coax on the
ground somewhat near a multiple of a halfwave. Typically something in the
range of 125 or 150 feet, give or take, can have this VOLTAGE node, high Z
point at one or both of the typical common mode blocking points.
If one models this literally, and puts an EZNEC source on the shield, then
you need common mode blocks in the 100k+ order of magnitude to keep noise
down low enough to protect a non-amplified pennant antenna. The reason the
TX FCP + isolation transformer is so quiet is that the UNCONNECTED windings
of the transformer have only the capacitance between windings as a through
path, and at the low 160m frequency the isolation is in the half megohm
range against a 75 ohm-ish shunt to ground at amp or RX.
So from where I'm setting, transformer ISOLATED preamps at EACH RX antenna
need to be PROVED OUT for naturally lossy RX antennas, rather than the
other way around, and you may need to run a separate DC lead to the preamp.
I'm not saying they CAN'T be proved out, it's just that the beginning
assumption needs to be that they are needed UNTIL it's proven they aren't
need.
I think that what is amiss is our perceptions about how quiet we think coax
shields are. Apparently coax shields are just plain gawd-awful noisy, and
must be assumed to be just plain gawd-awful noisy in the planning and
construction stages.
And I still get correspondence where the writer thinks they can evaluate RX
antenna performance by A/B tests and how loud the desired signal is.
Signal to noise cannot be done with the human ear and an A/B switch.
Nobody's ear is that good. Borrow the equipment, do the work, get it
right. Good 160 RX is a nasty, technical, unforgiving business with a mean
mind toward deception.
73, Guy.
On Fri, Jan 20, 2012 at 12:08 PM, Don Kirk <wd8dsb@aol.com> wrote:
>
> --------------------
> N4ZR said : I have a 20 dB ARR preamp", "My choice is whether to put it at
> the antenna end, incurring the added complexity of sending 12V DC to it via
> the coax, or to put it in the shack."
> --------------------
>
> Per the ON4UN book, "In most cases you can put the preamplifier in the
> shack. The signal loss in the feed line is a loss that affects both the
> signal and external noise. That means that the loss in the feedline does
> not affect the S/N ratio."
>
>
> I personally have 3 point fed pennants that are very small (51.6% the size
> of full size pennants), and therefore their gain is around -46dbi, and my
> preamp is located in the shack (W1FB slightly modified preamp). I've done
> some simple tests with my preamp out at the feedpoint versus in the shack
> and I personally was unable to detect any difference in S/N performance but
> my measurement system was not highly sophisticated.
>
> My feedline is 160 feet of RG58/U, and I intentionally have no breaks (no
> connectors) in this feedline (one solid run of feedline from the connector
> on the back of my preamp to the primary of my transformer which is located
> at the antennas (my feedline is soldered directly to the transformer
> primary), and I did this to eliminate any and all weather related connector
> problems.
>
> I only use one transformer to feed my 3 pennants, and I do switch the high
> impedance side of the transformer (the transformer secondary), and I switch
> both ends of the secondary (mandatory for multiple point fed pennant
> systems).
>
> I have 14 turns of my coax run through 3 stacked 2.4" O.D. 31 mix cores to
> help prevent common mode current, and this choke is located approximately
> 25 feet away from the base of multiple pennant feedpoint. Also have 14
> turns of my relay control cable (CAT 5 cable) running through 2 stacked
> 2.4" O.D. 31 mix cores which are located on the ground below my antenna
> feedpoint.
>
> 73's
> Don (wd8dsb)
>
>
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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