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Author Topic: Radio behavior if a feed wire is too small?  (Read 2352 times)
KE5TJT
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« on: February 19, 2014, 11:53:55 AM »

I used some old wire as feed wire for my radio installation in my car. The wire was never used but I think it was just speaker wire, and small wire at that. I was trying to adjust the new antenna and noticed that no matter what, I can only get 5 watts out of the radio whether I set the power to high, medium, low etc. But, if I take this radio and go to my truck, which has the exact same antenna but a larger power supply feed wire, I get full 20 watts on the setting of 20. (no need to go to 50, I can tell it's working fine.)

My feed wire might be too small, the question I have is if you know what it would behave like if it is indeed too small. I'm showing normal voltage at the radio, but of course wire size effects amperage, not voltage. I thought if the wire was too small it would simply get hot, not keep the radio from transmitting more than 5 watts. Any help would this is greatly appreciated.
« Last Edit: February 19, 2014, 11:56:42 AM by KE5TJT » Logged
WB6BYU
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Posts: 13341




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« Reply #1 on: February 19, 2014, 12:03:09 PM »

One simple check is whether the dial lamps dim when you transmit - if so, that's a good
sign to check the voltage drop in the wire.

What matters is measuring the voltage right at the radio while transmitting,
because that is when you are drawing the most current.  Best to do this into a dummy
load, however, as many digital meters will respond to stray radiated RF and not give an
accurate reading.

Testing on the dummy load also confirms that the problem is not due to the antenna tuning.

The problem could be voltage drop in the wire, or a poor connection to a fuse, ground lug,
battery terminal, etc.
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AA4PB
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« Reply #2 on: February 19, 2014, 12:08:38 PM »

If the wire is too small, the voltage **at the radio** will be too low (less than 12V). Smaller wire has more resistance. More resistance will drop more voltage for a given current. This is Ohm's law. E = I * R

You need to check the voltage at the radio while the transmitter is keyed and putting out maximum power.

Heating of the wire is one concern, but voltage drop is the other. The charts that give maximum current vs. wire size are based on an acceptable voltage drop and acceptable heating.
 
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KI6LZ
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« Reply #3 on: February 19, 2014, 12:17:53 PM »

Not sure if radio will fold back power due to load mismatch, if it is reducing power problem is with antenna.
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AA4PB
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« Reply #4 on: February 19, 2014, 12:24:05 PM »

If the radio is not getting enough voltage during transmit, the power output will be low. Low power doesn't necessarily have to be an antenna SWR issue.
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KI6LZ
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« Reply #5 on: February 19, 2014, 12:34:58 PM »

Just seems to me if the power is that low the radio is probably distorting and operating well below 12 volts. Surprised it isn't going flaky.
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K5LXP
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« Reply #6 on: February 19, 2014, 07:20:48 PM »

I thought if the wire was too small it would simply get hot, not keep the radio from transmitting more than 5 watts.

You didn't say how long the speaker wire is or what gauge it was, but what's likely happening is a combination of both current limiting and dissipation.

Whatever the zip cord is dissipating, it is doing so over the entire length of the wire so the temperature rise is relatively small.  Plus because it is limiting the power of the radio, the current is diminished too.
 
Good bet if you measure the voltage at the input of the radio you'd see more than half a volt drop when you key up.  You've proven the radio works so you've pretty much isolated it to the cable.

Mark K5LXP
Albuquerque, NM
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KE5TJT
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« Reply #7 on: February 20, 2014, 05:50:54 PM »

Yes I did. I made another cable in a larger AWG size and I got 40 watts at roughly 12.3 volts, that should be fine. This little Tram antenna I'm having trouble tuning. I can get the SWR's down to 1.7:1 on some 2m freq's and 1.1:1 on others, but I can't get it low across the board. Isn't 1.7:1 ok?
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K5LXP
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« Reply #8 on: February 20, 2014, 08:23:46 PM »

A 5/8 wave will be under 2:1 across the band, so it sounds like you're there.  An analyzer would help you nail the lowest point but 1.7 is just fine.


Mark K5LXP
Albuquerque, NM
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