Topband: 50 ohm direct burial coax cable

[Tom W8JI]

> You bring up some good points. I got a couple of hundred feet of Flooded 
> Commscope 75 ohm "RG-11" type at a good proce. I don't have the part 
> number handy. I've been very happy with it at 100 watts. I'm thinking of 
> running 500 watts. The center conductor is copper clad steel. I'm 
> concerned about RF current losses because of the skin effect on 160 
> meters. Does anyone have any experience with this type of CATV cable at 
> high power?
>
> A 1:5 maimatch may be an issue at the amplifier but I guess I can place a 
> L matching circuit between the amp and coax to get the SWR down.
>

The worse case SWR of a 50 ohm system with 75 ohm cable isn't 1.5:1 when 
normalized to 50 ohms. It is 2.25:1.  1.5*1.5 = 2.25

A 50 ohm load with 1/4 wave of 75 ohm is 112.5 ohms, and that is 2.25:1. 
This is why the cable needs to be 1/2 wave long, so impedance is back around 
50 ohms. If you are unlucky and pick an odd 1/4 wave, and the load is 50, 
the input SWR is 2.25 in the lossless cable case at the radio.

Also, on longer cables, the low SWR bandwidth is narrower. This is because 
the cable is multiple 1/4 wave sections in series. If there are ten of the 
1/4 waves making up a 2.5 wave long cable, and if the frequency changes 5%, 
the total length error for all the sections is 50%. This narrows bandwidth, 
and reduces the depth of the magical low SWR point.

If the antenna is a bit higher that 50 ohms things get better, and if it is 
a a bit lower than 50 ohms things get worse fast.

With so much cheap 50 ohm cable pulled from cell sites around, it is far 
easier around here to get 50 ohm rather than 75.

Years ago I used 75 ohm because it was everywhere for free. Now I use 50 ohm 
heliax and don't regret it one bit. Small used lengths pulled from 2-way and 
cell sites are free or a very nominal cost

73 Tom