[VHFcontesting] Coax

[James Duffey]

Rich - You wrote:
 >What would be the best coax to use for 2m SSB? I have RG-58 but I  
have been told that it's lossy on 2M SSB.
You got good advice from nearly everybody. As others have said, it  
depends. Three big considerations are how much loss can you stand, how  
long is the feed line run, and how much are you willing to pay for  
lower loss?
One dB is pretty much the smallest change in signal strength that one  
can discern repeatably, so some put this as an upper limit on feed  
line loss. The problem with this thinking is that if one repeats it  
with all sources of loss in a system overall loss quickly adds up.  
Perhaps a half dB is a better choice to allow for other losses. Less  
loss is better if one can afford it, but for most work much less than  
0.5 dB is overkill.
So lets look at Marshall's Times Chart, but this time with the feed  
line length that causes 1 dB loss:
Transmission line type	50	144	220	440
RG58			33'	17'	14'	9'
RG8X			40'	21'	17'	11'
RG213			60'	36'	28	19'
LMR400			110'	67'	55'	37'

Halve these lengths for 0.5 dB loss. So, unless you have  very short  
runs of feed line, yes RG58 will be excessively lossy.
What is a dB worth? 100 feet of LMR400 costs $75 at the RF COnnection.  
The cheapest RG58 at the RF Connection costs $18/100 ft. LMR400 has  
1.5 dB loss per 100 ft, RG58 has 6.2. So you have bought 4.7 dB less  
loss for $57 more, or about $12 per dB.
How does this compare to other ways to ourchase dBs? If you are  
running a typical 25 Watt rig, a 160 Watt linear will get you  8.1 dB  
at a cost of $355 new at R&L electronics, a cost of $44 oer dB. That  
will take care of the transmit side, but to get over that loss on  
receive, you will need to add a mast mount preamp; ARR's 25 Watt mast  
mounted job will set you back about $190, for perhaps a 3 dB noise  
figure reduction that you get with the LMR400 and no mast mounted  
preamp at a cost of $64 per dB.
Well that is not a rigorous analysis, but you can do one for your  
situation. You can get the feedline cheaper, buy a used amp and so on,  
but the cost of feedline is usually the most cost effective way to  
reduce system loss on both the receive and transmit end for typical  
amateur installations. Go to lower loss feed line before you try a  
linear. Then add the linear. - Duffey


--
KK6MC
James Duffey
Cedar Crest NM