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I have a Kenwood TH-D72 and I just got an old Radio Shack HTA-20 2m amplifier to use with it. They're both powered from a sizeable 3S2P Li-Ion battery pack and they work great together for the most part, but I recently noticed something odd. I connected a meter to measure the current draw of the amp, and with it turned off it still seemed to be drawing about 100mA. After a bit of head scratching as the amp has a physical power switch and so shouldn't draw anything when off, I concluded that when they're both connected to the same battery the radio must be getting a much lower resistance connection to the power supply ground/negative through the coax braid and the 12 gauge wire I've run to the amp than through its own comparatively dinky power lead; when I unplug the radio from the shared external battery or disconnect the coax from the radio to the amp, the current draw at the powered-off amp goes away.

Using the coax as a power supply lead just doesn't seem like a good thing to me, but is this really a big deal? The amp starts acting up when the battery gets below about 10.7V in this configuration, which is what got me hooking up meters and testing things in the first place, but it does the same thing when the battery gets a bit lower even when the radio is running on its internal battery, so I think this amp just doesn't handle undervoltage well and the extra voltage drop to the amp when its cabling is also partly powering the radio (which itself pulls a couple of amps on TX) causes it to misbehave sooner.

So, do I need to stop the radio from using the coax braid as its primary negative power supply connection and if so, how? Some sort of balun/unun/other transformerish isolator?

Thanks!

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    $\begingroup$ Did you measure the 100mA on the +ve lead or -ve? $\endgroup$
    – vu2nan
    Apr 15, 2020 at 5:13
  • $\begingroup$ I measured it on the negative lead, but actually, that's a good question, because if I were just using my multimeter I'd have been measuring on the positive and probably never even noticed the drain, but the meter I'm using is a panel meter which combines voltage and amperage and its wiring diagram is pretty specific that the ammeter shunt must be wired on the negative side, so that's where I was measuring. It's this meter: amazon.com/gp/product/B07PLQGCZS $\endgroup$
    – MoTLD
    Apr 15, 2020 at 7:41
  • $\begingroup$ Thanks for the info, OM Rob. Does the HT go off when the coax to the amp is disconnected? $\endgroup$
    – vu2nan
    Apr 15, 2020 at 13:09
  • $\begingroup$ How, exactly, is the coax shield acting as a "ground" connection? The equipment configuration you describe seems to be entirely isolated from "ground." $\endgroup$
    – Brian K1LI
    Apr 15, 2020 at 15:21
  • $\begingroup$ @vu2nan, the HT works fine either way. $\endgroup$
    – MoTLD
    Apr 15, 2020 at 18:15

3 Answers 3

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If I understand the description correctly:

schematic

simulate this circuit – Schematic created using CircuitLab

If that's all it is, I wouldn't say it's any big deal. Whenever you have multiple paths to ground, current will split between them. It's these currents that keep "ground" approximately the same voltage everywhere: an essential assumption in the design of most electronic equipment.

Some DC on the coax won't cause any issue. In fact putting a DC bias on coax is a common way to power an active antenna.

It could be the power leads for the radio are simply too light. The ground lead may not be a problem since the coax provides a low impedance connection, but the positive lead is equally small. Try eliminating the coax by attaching a dummy load to the radio. Then while transmitting, compare the battery voltage to the voltage at the radio's terminals. It shouldn't drop more than a volt, preferably less. You can also multiply that difference times the supply current and calculate the power wasted in the power leads, which might be of concern if you're trying to maximize the battery life of this station.

Out of an abundance of caution, I should also mention the possibility that this current has some more dangerous origin involving a fault in your mains wiring. You didn't describe anything not powered by the battery, but if you have for example a connection from the radio to a computer, and there's a ground fault in the computer, that ground current you are measuring could be the only thing keeping the chassis of your equipment from being an electrocution hazard. It's unlikely but possible, so be careful.

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  • $\begingroup$ You understand the description perfectly, that schematic is exactly how it's wired, with assumptions for the resistance values anyway. Thanks for illustrating it and for the reassurance that I might not have a problem at all. As far as the voltage drop in the radio's power leads, that is a concern of mine as well just from a battery life standpoint, and I'm already using larger leads than came with the radio's own supply, but I intend to beef them up further. There's just that compromise between better conductivity and the point at which bulky stiff cables to an HT become quite inconvenient... $\endgroup$
    – MoTLD
    Apr 15, 2020 at 23:55
  • $\begingroup$ Also, I'll keep in mind the faulty mains ground possibility and do some more testing with and without connections to the computer and the battery charger. $\endgroup$
    – MoTLD
    Apr 15, 2020 at 23:57
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An anecdote: I had problems with my station which included computer-radio interconnections; some of my devices (RTL-SDRs) would lock up when I powered on my Yaesu FT-897. I eventually tracked this down to the power-on inrush currrent of the radio passing through coax shields and the chassis of the antenna switch that both were connected to.

I solved the problem by replacing the fuse (an element with resistance, after all) in the negative power lead of the FT-897 with a jumper (keeping the fuse in the positive side). This way, the negative power wire's impedance has been reduced compared to the paths not intended for power.

So, the above is an example of one kind of problem that can happen, and a general precaution to take:

  • Make sure your power wires are fat and securely connected.
  • Make sure there is no way to have a positive connection without a negative.
  • Don't use an ammeter or current shunt on the negative lead, except where there is no other negative-side path (e.g. the battery terminals, or an otherwise-isolated load, but not interconnected components). Not only will it cause shield currents, it will read inaccurately since some current is bypassing it.

Adding galvanic isolation by a transformer ("voltage balun" in this context) could work, and will completely eliminate rather than reduce the stray current, but would be an expensive and heavy component (typical baluns are "current baluns", which do not isolate, because that way they can avoid needing a magnetic core sized for the full RF power being carried). I've never heard of someone using one.

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  • $\begingroup$ Thanks for answering, lots of good info here! I thought the problems with the amp at 10.7V might be related to this DC on the braid situation, but the more I think about it I think it's just the extra voltage drop and the amp not being able to handle such low voltage in general. I will definitely keep in mind the advice to keep any negative-lead current shunt between the battery and all the equipment instead of in line with any particular piece of equipment to avoid misreadings, especially since that's how my panel meter must be rigged. $\endgroup$
    – MoTLD
    Apr 15, 2020 at 20:41
  • $\begingroup$ Also, a 1:1 voltage balun was the isolation solution my research turned up as well, if I conclude that it even needs isolation, but how much loss are we likely looking at? The narrowbanding isn't a big deal so long as it can still cover the FM part of 2m since the amp only does 2m FM, but I would prefer to avoid excessive loss, especially on receive. $\endgroup$
    – MoTLD
    Apr 15, 2020 at 20:46
  • $\begingroup$ @MoTLD I'm afraid I don't know much about the performance of transformers at RF — I just know that they aren't used routinely for baluns even though it seems like the obvious choice — ah, because you'd need a comparatively big core to pass the RF power without saturating, if I remember better. I'll reword that. $\endgroup$
    – Kevin Reid AG6YO ♦
    Apr 15, 2020 at 21:53
  • $\begingroup$ It would be easier to float the power supply, rather than the RF signal. Though I'd want to identify the source of the issue first. $\endgroup$
    – Phil Frost - W8II
    Apr 15, 2020 at 22:58
  • $\begingroup$ I take that back, I guess it's not so easy its all battery powered. $\endgroup$
    – Phil Frost - W8II
    Apr 15, 2020 at 23:05
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Should a 'a not so good contact' at the HT's external jack/plug be ruled out as a contributory factor, the only way to avoid DC current in the coax braid would be to have a separate battery pack for the HT.

The moot point is whether DC current in the braid does matter.

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  • $\begingroup$ Thanks for answering! I've ruled out a bad connection at the HT, as it runs right down to the 9.7V or so minimum it seems to require when it's the only thing connected to the battery, which is pretty good for something expecting 13.8V, so I don't think there's any excessive voltage drop in its leads or connections, though I'm working on heavier gauge cabling for it to squeeze that much more on-time out of each battery charge. Can't go but so heavy on an HT before it starts getting in the way, though... $\endgroup$
    – MoTLD
    Apr 15, 2020 at 20:38
  • $\begingroup$ Anytime, OM Rob, and thank you for all the information. Yes, there's a limit to increasing the cable size. The stouter cable wouldn't be a big hindrance should you be using a speaker/mic. $\endgroup$
    – vu2nan
    Apr 16, 2020 at 2:25

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