Haven't you noticed the OP disappeared once his question was answered and your techie mumbo jumbo impresses nobody but yourselves? (;->)
But, the OP did get his questions answered. And, as far as I am concerned, once the best attempts have been made to answer a question there is nothing wrong with the conversations meandering on into other related issues. Hijacking a thread before the answers are out there is bad form, but continuing the discussion past that point into other areas after the basics have been addressed is just a logical progression.
Coax electrical length does come into play but I'm pointing out pure loss from operating the coax in a very high VSWR system. A G5RV has upwards of 20:1 or worse mismatch on some bands at the end of the twinlead and feeding coax at that point is a very lossy proposition.
prcguy
No one doubts or should question that there is additional significant loss in very high VSWR situations. This loss is on top of and additive with the original loss value (matched-line loss) of the coax. This means that the lower the matched-line loss value, the better, my statement was that the examples you were giving, more than 10 dB on a 100 foot run, have to include other factors. “Adding 100ft of RG-58 can easily cause 10dB or more loss on some bands even though RG-58 is only rated for a few dB loss at HF.” My general contention is, and was, that the RG-58 would not be hugely different from RG-213 in such an application, as long as we are talking about low power and HF. Both feedlines are going to have SWR induced losses, although they will be different.
The additional line loss due to VSWR is fairly well understood. The current edition of the ARRL Antenna Book even has a nice little section on it and a chart to help calculate what this loss can be. They also have the math present if you would rather work it out yourself, but I trust the chart. Chapter 24, page 10, figure 14.
And what all that comes down to is that with a 20:1 VSWR the difference in loss of RG-58U (with a matched-line loss of 2.5 dB / 100 foot at 30 MHz) and RG-213 (with a matched-line loss of 1.2 dB / 100 foot at 30 MHz) will not be large. In fact the RG-213 will have about 6.4 dB total (matched-line loss plus SWR induced additional loss) while the RG-58 will have about 8 dB total. Or about 1.6 dB difference between them. Higher VSWR will raise all of these values, including the delta between them, but the delta is going to be the smallest change.
There are many good reasons to run good, high quality, coax. But, RG-58U often gets hammered far harder than it should. For some applications it simply is not bad. It might not be as good as other feedlines, but sometimes the difference is pretty small.
T!
Mohave Desert, California, USA
I'm being sarcastic but the losses are very high even on HF -- 50-60ft is a substantial length and losses do happen. This is especially important if you're transmitting. Not so much if it's just a listening antenna.
