[TowerTalk] Measuring Coax Loss

[Tod -ID]

Jim and Rick:

Tell me again how I can productively use the more precise information [0.6
versus 0.62 dB loss in 208 feet at 3 MHz.] in constructing my antenna feed
line system. 

I am afraid I have been oblivious to this aspect of things and may have
missed an opportunity to have improved my station.


Tod, K0TO


 

> -----Original Message-----
> From: towertalk-bounces at contesting.com 
> [mailto:towertalk-bounces at contesting.com] On Behalf Of 
> Richard (Rick) Karlquist
> Sent: Thursday, February 11, 2010 2:22 PM
> To: Tower and HF antenna construction topics.
> Subject: Re: [TowerTalk] Measuring Coax Loss
> 
> I also have an AIM-4170 network.  It is considerably better 
> than the MFJ-259B.  It would have allowed additional 
> resolution and accuracy in a reflection mode measurement of loss.
> 
> If you still want to do a through measurement, it would 
> probably be better to use a power meter than a spectrum 
> analyzer at the far end, since a power meter is more accurate 
> than a spectrum analyzer, plus being smaller and cheaper.
> 
> It is also worth noting that when you start talking about 0.1 
> dB or even 0.01 dB differences in insertion loss, you have to 
> start worrying about whether the characteristic impedance of 
> the coax is exactly 50 ohms or not.  Otherwise, there will be 
> a "mismatch error".
> 
> Notice that at 50 MHz the loss was 2.07 dB and at 3 MHz, the loss was
> 0.62 dB.  If there were no "low frequency effects", the loss 
> at 3 MHz should have been about .50 dB.
> 
> Rick N6RK
> 
> Jim Brown wrote:
> > N6RK suggested that the MFJ259B would be a good way to measure coax 
> > loss with a reflection measurement at one end only. I 
> responded I was 
> > looking for more precision than that. Several years ago, I 
> carefully 
> > calibrated my 259B using W8JI's procedure. Today, I compared loss 
> > measurements of a 208 ft length of Commscope 2427K (the 
> plenum version 
> > of 3227) made both with my 259B and with my HP 
> generator/spectrum analyzer setup.
> > 
> > The HP data has one digit better precision -- the spectrum analyzer 
> > readout is 0.01dB, while the 259B reads 0.1 dB. Within 
> those limits, 
> > the MFJ yielded results that were usually within rounding 
> error of the 
> > HP setup between 10MHz and 150 MHz, which is where I 
> stopped measuring 
> > (because that's the highest frequency I cared about for my 
> > applications). For example, at 50, 100, and 150 MHz the HP numbers 
> > were 2.07 dB, 2.84 dB, and 3.37 dB; the MFJ yielded
> > 2.1 dB, 2.8 dB, and 3.3 dB. At 3 MHz (the lowest frequency I could 
> > measure with the MFJ), the numbers were 0.62 dB and 0.6 dB.
> > 
> > For obvious reasons, you really need the greater precision 
> of the HP 
> > setup to get good data for frequencies and lengths where 
> the loss is relatively small.
> > 
> > 73,
> > 
> > Jim K9YC
> > 
> > 
> > _______________________________________________
> > 
> > 
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