[TenTec] SWR is a DUMB Indicator of Antenna Performance

[Bob McGraw - K4TAX]

Good explanation and I do see the reference.  Thanks.

73
Bob, K4TAX


----- Original Message ----- 
From: "Steve Hunt" <steve at karinya.net>
To: "Discussion of Ten-Tec Equipment" <tentec at contesting.com>
Sent: Monday, September 15, 2014 4:54 PM
Subject: Re: [TenTec] SWR is a DUMB Indicator of Antenna Performance


> At HF the losses in most transmission lines are copper losses, not 
> dielectric losses; so the losses are directly proportional to the square 
> of the current.
> 
> Picture the current standing wave along a mismatched TL with an SWR of, 
> say, 4:1. At the current maxima the current will be twice what it would 
> be in the matched case for the same power, and at the current minima it 
> will be half what it would be in the matched case. So the cable losses 
> at the current maxima will be 4 times what they would be in the matched 
> case, and at the current minima they will be one quarter what they would 
> be in the matched case.
> 
> Integrated over a long length of line the net effect is more loss than 
> in the matched case; but for short lengths of line that isn't 
> necessarily true.
> 
> Take as an example calculating the losses in 2ft of RG58 wound on a 
> ferrite core to form a 1:1 balun. Feeding a matched 50 Ohm load with 
> 100W the differential-mode current in the coax will be 1.4A; But if we 
> now swap the 50 Ohm load for a 200 Ohm load (SWR=4:1), the current at 
> the load end of the coax will drop to 0.7A and the power loss at that 
> point in the cable will be one quarter what it was in the matched case. 
> Further back from the load, the current in the cable will rise due to 
> the standing wave pattern, and the difference reduces; however for short 
> lengths of cable the integrated loss can be less than the matched loss.
> 
> At, say, 21MHz the matched loss for 2ft of RG58 is 0.036dB, but with a 
> 200 Ohm termination it drops to 0.022dB - about 40% less.
> 
> The converse happens if the termination is a low resistance, because the 
> current is then higher than in the matched case. With a 12.5 Ohm load 
> the loss rises to 0.131dB - significantly more that in the matched case.
> 
> So there we have an example where the cable losses for the same 4:1 SWR 
> are very different - in one case higher than the matched loss and in the 
> other case lower than the matched loss. Therefore any loss calculator 
> which asks for SWR as an input parameter rather than actual load 
> impedance is making assumptions which might not be valid! And any table 
> that simply shows "additional loss" due to some particular SWR should be 
> treated with caution.
> 
> Hope that's clear,
> Steve G3TXQ
> 
> 
> 
> 
> On 15/09/2014 22:13, Bob McGraw - K4TAX wrote:
>> OK, I'll fall for it, how does a cable have a negative dielectric loss 
>> figure?
>>
>> 73
>> Bob, K4TAX
>>
> 
> _______________________________________________
> TenTec mailing list
> TenTec at contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
> 
>