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@contesting.com
> [mailto:towertalk-bounces@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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> >
> >
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