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Topband: RX Power over Coax

To: Dennis W0JX <w0jx@yahoo.com>, "topband@contesting.com" <topband@contesting.com>
Subject: Topband: RX Power over Coax
From: Lee STRAHAN <k7tjr@msn.com>
Date: Tue, 6 Oct 2020 07:15:56 +0000
List-post: <mailto:topband@contesting.com>
 Hi Dennis and top band interested folks,
   The very first amps I made for myself were using an external power wire. 
Using external power required some pretty heavy choking on each end of that 
added wire. I started experimenting with power over the signal coax. I could 
not see or measure any difference in noise floors with usually new components 
including coax. After a number of years' experience I now believe that if there 
was a weakened connection that would allow micro arcing then that same 
weakening would cause a perturbation in the operation of the array also. This 
resultant lack of performance would signal the operator that something was 
wrong. All the elements in most of my designs are required to operate and feed 
signal continuously to the combiner regardless of the direction selected so all 
would be susceptible. It's my opinion as I have not witnessed any arc sounding 
interference or noise floor increase that I could not identify as external to 
the array. Keep in mind I have not done yearlong in depth projects to verify my 
observations or real levels of possible noise. I am sure a fully loaded 
environmental laboratory could do this but so far I cant.
     If one desired to power the amplifiers externally any noise current that 
would be developed on a single wire must then also flow into the power terminal 
where it would encounter a generally large filtering capacitor across the power 
rails of the amp and its ground side. The noise current would then flow right 
through to the amp ground terminal and be impressed upon the array ground rod. 
This also is not a desirable thing as the ground rods exhibit impedance and a 
noise voltage could then show up as added to the element signal. Series 
inductance in this line can help. So it appears the best answer to this is to 
use a second coaxial cable or 2 wires to supply the voltage and power ground to 
the amp. Now with both + power and ground wires feeding power to the amp one 
can use common mode filtering to remove the noise flowing to ground and an 
additional inductance to decrease any noise voltage on the center of the coax 
carrying the plus power voltage.
   Its true, I have built a couple versions of Hi-Z amps with RG-6 external 
power connections for Tim at K3LR. He has serious common mode filtering on 
these power lines as well as the signal lines and I suspect serious filtering 
on the supply voltage as well. There may in my mind be some advantage of 
external power in keeping multiple transmitter signals off the array power as 
well. 
   Its also true that I have a small circuit board I can install on the back of 
a second coax connector on the New version SMD amps. The board offers 5000 ohms 
of common mode protection on 160M and a series inductance for the center wire 
providing 1 mHy inductance giving some isolation for power supply or induced 
center conductor noise. No need for expensive giant ferrite cores on the coax 
lines. Common mode protection is built in to both the coax power input and the 
coax signal output.
    So due to lack of interest I have not championed the idea of providing this 
external power capability. I think the reputation of success with Hi-Z and its 
power over coax stands for itself as to the existence of the problem at Hi-z or 
the public would be letting us all know something is wrong. Trust me on that. I 
cant speak for any other designs.
    In addition to this noise incursion possibility I have uncovered another 
possibility of micro arcing unrelated to the coax. As it stands most of these 
arrays are powered from an in shack power supply. With some of these arrays 
drawing 1/2 an amp or more if using a wireless remote relay setup. This causes 
a significant voltage drop on power wires leading to the array. With the power 
supply grounded at the shack and the Hi-Z amps grounded at the array elements 
there is often a voltage difference between shack ground and element ground 
caused by the power supply current flow. This voltage difference causes a 
current flow in the ground rod. In more than 18 years of operation I will soon 
have to replace my original Steel Tee post mounted elements as the ground end 
of the posts has been eaten away by what I assume to be electrolysis. My theory 
is that there could be noise developed on the ground rod at the element caused 
by this voltage dropped between the shack and the array causing the 
electrolysis. Noise on the ground rod shows directly up as received signal. My 
new version of SMD amps have been designed so that DC voltage will not be sent 
to the ground rod at the elements. If anyone has experienced noise as a result 
of the power over coax with a Hi-Z array I would welcome information of the 
circumstances. Come to think of it,  I believe I have a new amp with external 
power laying around here. I will make it a point to see if I can identify any 
noise difference between amps with and amps without External power. This will 
make an interesting experiment even if its inconclusive.
   I can modify the new amps for this external power on direct to me special 
request..
73

Lee   K7TJR
Hi-Z Antennas




Subject: Topband: RX Power over Coax

Regarding the issue of voltage over coax to power the preamps used at the base 
of short verticals, Lee, K7TJR, developed an alternative power feed system for 
his new SMD preamps using a separate RG6 coax. Apparently, K3LR is using such 
an approach to avoid sending voltage down the coax in his 8 element HiZ array 
at his contest station.
DXE has begun selling the new version 2 preamps but the description in their 
catalog does not mention this separate power feed system. The older HiZ 
preamps, and the YCCC preamps, can probably be modified to the voltage with a 
separate wire from the combiner box to each vertical antenna preamp.
One question I have with this approach is whether an external power wire would 
also need a common mode choke on it.  Perhaps Lee could offer some opinions on 
this alternative power approach.
73, Dennis W0JXMilan, OH
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