[TowerTalk] Coax Losses on 160 and 75?

[Roger (K8RI) on TT]

Although we tend to think of using "big" coax at UHF, or even VHF, it 
appears to me that the lower frequencies benefit as much, or more than 
UHF, but for different reasons.  I also prefer the larger coax because 
of the rapid rise in SWR as you move away from resonance compared to 40 
meters and up.

As for cost, I see little difference between the LMR cable and other 
name brands.  The biggest difference in going to LMR 600 is the cost of 
connectors.
LMR-400 is notably lighter than all copper.

As for measuring loss, I prefer using an AIM that gives me all I want to 
know and more than I need.  The problem with SWR meters is accuracy.  
How much do they change with temp.  I have a pair of birds, that are 
accurate to 5% of full scale.

Both can be hooked to the input to the coax and calibrated to read the 
same. It pays to do the measurements and then swap the meters and 
measure again.  You really need a digital readout, or measure in the 
area where the scale is expanded. That's an area where accuracy is low, 
but if calibrated to a known power you are comparing against two known 
values.

73

Roger (K8RI)


On 8/6/2016 Saturday 1:00 AM, Jim Thomson wrote:
> Date: Fri, 5 Aug 2016 09:35:17 -0700
> From: Jim Brown <jim at audiosystemsgroup.com>
> To: "towertalk at contesting.com" <towertalk at contesting.com>
> Subject: Re: [TowerTalk] Coax Losses on 160 and 75?
>
>
> Nothing "however" about it, Roger. Of course it's all skin effect, and
> copper-coated or clad onto steel has much greater loss than solid or
> stranded copper at these frequencies.
>
> BUT - there's a big difference between copper-clad Aluminum and
> copper-clad steel. RF resistance of copper-clad Al is NOT significantly
> different from solid/stranded copper at any frequencies where we are
> likely to use it.
>
> Yes, at HF the most reliable comparative of cable loss below UHF is the
> total resistance of their conductors at the frequency of interest. For
> solid copper conductors, DC resistance can be compared, because
> resistance at RF is a multiple of the DC resistance. For copper-clad Al,
> the resistance of a solid copper conductor representative of the DC
> resistance of the center. So -- there's nothing wrong with using LMR400
> and other cables with Cu-clad Al center on the lower HF bands, except
> that they are NO BETTER than a lower cost coax that provides the same RF
> resistance. That's why my table in k9yc.com/Coax-Stubs.pdf lists coax
> cables in order of their DC resistance, and why the last column in the
> table is cost.
>
> 73, Jim K9YC
>
> ##  How are u measuring loss at a given freq ??   Do u use the online calculators,
> like from Times microwave, where u input coax type, length, and freq ?   Or do u
> just do it the old fashioned way..and install a calibrated wattmeter on each end of the coax ??
>
> ##  I like the wattmeter method, then I know it’s the ....real deal.  Just install a 50 ohm dummy load on the
> far end..just after the  2nd wattmeter.   Then you can test from 160- UHF.   Then you get real results.
> This assumes the pair of wattmeters are calibrated for the  freqS  to be tested.   They dont even have to be
> calibrated, they just have to read identical...when both are wired nose to tail, in series, at the xcvr /amp.
> I wire two in series 1st.....then test to ss how close they are  across  XXX bands /freqs.    Ok, then remove one of em
> and install at far end of coax.
>
> ##  You folks will be in for a rude awakening if you try this stunt.
>
> Jim   VE7RF
>
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-- 

73

Roger (K8RI)


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