[TowerTalk] measuring coax loss with an MFJ-259B

[ersmar@comcast.net]

Pete:

     You are probably correct:  Using your 75 Ohm coax hunk at other than 28.000 and 28.875 (representing 32 and 33 half-wavelengths, respectively, of your fundamental freq of 1.750 MHz) is putting your measurement into the whacky world of transmission line mismatches and the Smith Chart.  That is, your measurement technique is valid ONLY at 28.000 and 28.750 MHz (exact half-wavelength multiples.)  Away from these freqs and you see the complex impedance of your far-end coax cable PLUS whatever the length of 75 Ohm cable looks like.

     This error exists on lower freqs, too, as you probably know.  But the freqs at which the errors would be reproduced are outside the Ham bands - not of much interest to you, I suppose.

73 de
Gene Smar  AD3F



> I have just been trying to get a baseline loss measurement on my two feedlines 
> from shack to tower, using the MFJ-259B in Coax loss mode.
> 
> According to the manual, the unit can be set to any frequency within its range, 
> and will read out a direct return loss number at that frequency, if the coax is 
> open at the other end.
> 
> In practice, I find that around 28 MHz, for instance, the displayed loss value 
> for one feedline can be anywhere from .9 to 2.1 dB, depending on the frequency.  
> The displayed SWR on the unit is at a minimum when the loss figure is maximum, 
> and vice versa.
> 
> I think I may know why, if not how, this is happening.  My feedlines both 
> utilize long runs of 75-ohm CATV hardline, cut to a half-wave multiple at 1750 
> KHz, and measured at 28 MHz.  On either end of the 75-ohm runs are runs of 
> 50-ohm RG-213 to the antennas and into the shack.  My theory in doing things 
> this way is that the 75-ohm will reflect the input impedance at the output, and 
> as a practical matter it seems to work rather well.  But my hunch is that 
> particularly at 28 MHz, relatively small frequency excursions can result in the 
> 75-ohm section translating to quite another impedance than the 50 ohms at the 
> input end, and that this in turn results in the changing "loss" numbers.
> 
> Do I have this right?  If so, is any of the numbers I'm seeing an accurate 
> reflection of the loss in the total composite feedline? 
> 
> 73, Pete N4ZR
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