Topband: RX Power over Coax

[Lee STRAHAN]

 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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