Trouble shooting GME AIS issue

[betwys1]

Originally Posted by Bycrick

You didn’t hurt my feelings. I just pointed out that measuring at anything at the top of the mast is more difficult physically, more risky to the equipment and unlikely to produce better results than measuring at the radio. Using cheap tools at the top of the mast to reduce the economic risk is a choice you’re more than welcome to make. But when you then advocate a VSWR of 1.6:1 or less, I’m a little curious how you judge that using a dummy load that doesn’t even spec what its VSWR is. My lab grade on is speced at 1.2:1, but my MFJ throw-away one is 1.5:1.

You’re certainly free to do things however you choose, but if you’re going to pass out free advice to people, you probably should be able to defend it.

I see I misjudged you. You were not asserting superior knowledge or equipment, but rather a genuine misunderstanding.
You believe that you can measure total system performance from the comfortable cabin or mast connector. That is what you assert above and it is incorrect. [I put your assertion in bold]
However, it IS true that masthead tests are more physically demanding, and incur a risk to equipment carried aloft.

So let's say you scored a bulls-eye on 2 of the 3 points, and a miss on the third. Let me know if you want me to flesh out that error.

Brian W

[Bycrick]

I certainly wasn’t asserting "superior knowledge" in any general sense. Just presenting another useful technique which you didn’t seem to know about.

I don’t think it’s just an assertion that my wattmeter and dummy load are better better designed, better built and more reliable and accurate than $50 worth of stuff bought off eBay.

I WAS asserting was that you CAN measure the total system performance from the base of the mast or at the radio.

If a piece of coax is reasonably matched to the antenna, the only effect it has on the measurement of the system (antenna plus cable) VSWR is the loss in the cable. Cable losses will make the VSWR look better than it actually is. But if you KNOW what the cable losses are, you can correct for the losses when you compute the system VSWR. The three parameters for the computation are forward power, reflected power and cable loss.

You can easily compute the cable loss by measuring the forward vs reflected power when the cable is open-circuited. The open circuit reflects essentially ALL the energy that reaches the open end of the cable. So the reflected power is equal to the forward power minus twice the cable loss.

If I had a roll of cable sitting on the cabin sole where I needed to know the loss, and both ends were easily accessible with the proper connectors, I’d use Wotname's method. It’s quick and simple. But if the cable was 500’ long and one end was inaccessible on the inside of the roll , I’d put a connector on one end of the roll and use my method. They both work. If you think my method doesn’t work, just Google "measuring loss in coaxial cable." Several of the first five results describe my method in more detail.

Good equipment let’s you measure things with reasonably certain accuracy. I just happen to have good equipment. So I’m not going to take MY wattmeter and dummy load to the top of the mast. Could I buy some cheap junk to take up the mast? Sure. But then my measurements are less accurate and reliable. Using the method I described, I don’t have to take anything up the mast except the tools to disconnect the antenna and the various sealants when I hook it back up. And I can use good equipment to make the measurements.

Do most people need a Bird wattmeter and a good dummy load? No. But a $15 dummy load and a $20 SWR meter aren’t going to be very useful. Maybe better than nothing, but maybe not, depending on how wrong they are. For testing VHF radios, you can buy a good 10 watt dummy load for about $40 and a decent power/SWR meter for $75-100. You’ll need a $60-80 dummy load to measure 25 watts.