I feed my normal Beverages with 320' of RG-58. and sometimes (at other
locations) feed a Beverage with a shorter cable. Never ever have I been
able to pull the antenna from the transformer between antenna and coax
and have quietness in the receiver. Always there is too much pickup
from AM BC stations (which I DX) and also noise.
So I'll agree with Guy that few people's coax feeds are as clean as they
think. For that reason, I'd put the preamp at the transformer so that
the receiver thinks there's more S in the numerator with the same N in
the denominator. Of course in a low noise environment, there will be
little or no difference.
The loss is immaterial to me. At .4 dB/100 ft at 1 MHz, 300 feet of
RG-58 has 1.4 dB loss on paper and measured results show somewhat less loss.
Chuck
On 1/20/2012 10:44 AM, Guy Olinger K2AV wrote:
> 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
>>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
>
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UR RST IS ... ... ..9 QSB QSB - hw? BK
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