The connectors themselves usually do not directly cause losses of any
significance. However at higher frequencies there is what is called
"mismatch loss". The connectors provide an impedance bump in the line and
cause some reflections. This raises the swr slightly and some loss is
attributed to swr loss. The greater loss usually occurs with a resultant
mismatch at the radio which is often a fixed output impedance. The radio is
no longer able to couple its full power to the line because of the mismatch.
This is quite noticeable at uhf and vhf. I suspect that this is the reason
that the author of the article quoted found the 2 db loss on vhf.
With a matching device at the radio the problem becomes hardly noticeable.
73
Gary K4FMX
> -----Original Message-----
> From: towertalk-bounces@contesting.com [mailto:towertalk-
> bounces@contesting.com] On Behalf Of Dale Martin
> Sent: Monday, November 20, 2006 8:19 AM
> To: towertalk@contesting.com
> Subject: Re: [TowerTalk] LMR400/BR400 questions
>
>
>
> What kind of losses do you expect to sustain with a couple of connectors,
> Gordon?
>
> I think you may have more to worry about with the feedline loss than with
> the insertion loss from a couple of connectors. LMR-400 at 1000MHz shows
> 4.3 db loss per 100 feet.
>
> Check this out...
>
> 73, dale, kg5u
>
>
> This article appeared in the July/August 1988 National Contest Journal
> and,
> before that, in the April 1988 TDXS Bullsheet newsletter.
>
> Coax Fittings Redux
> by Bill Schrader, K2TNO
> (Reprinted from the TDXS Bullsheet, April 1988)
>
> I've always fallen for the old adage that says that UHF coax fittings add
> losses and should be kept to a minimum. Recently, I had occasion to clean
> out my tool box (after ARRL Phone at NR5M) and found 5 right angles, 4
> barrels, 3 double-males, and several short lengths (1-3 feet) of RG-8X. I
> decided to test this adage by connecting up this wild assortment of coax
> fittings into a plumber's delight series arrangement and then checking the
> loss and SWR problems on several frequencies.
>
> The test set up consisted of my TS-930S or 2-m rig feeding a dummy load
> with
> a Daiwa power meter as the indicator. I measured RF power at 14, 28 and
> 144
> MHz using either rig wired directly to the load or when fed through the
> mess
> of fittings.
>
> The connections were made at random, simply to use up as many fittings as
> possible. When done, there were 17 male-female coax joints in the line,
> as
> opposed to two male-female coax joints when feeding the dummy load
> directly.
> Thus the results below show the additional loss due to the 15 coax joints.
>
> Frequency Attentuation (dB) SWR Change
> 14 Not Detectable Not Detectable
> 28 0.3 +0.1*
> 144 2.0 Not Measured
>
> *SWR was 1.1:1 with coax only; this value increased to 1.2:1 when the
> series
> of joints was added.
>
> The results show that UHF coax fittings themselves add negligible loss in
> the HF spectrum and are surprisingly good even at 144 MHz. Thus, for HF
> purposes I conclude that addition of right angles, jumpers, etc., does not
> add significant additional attenuation or reactance. Rather, it is
> certainly the coax length itself which causes the biggest losses.
>
> Caveats: Coax joints are deleterious for reasons other than their
> possible
> RF loss characteristics. These include:
> 1) The PL-259s are frequently assembled and soldered incorrectly.
> 2) UHF fittings are not waterproof.
> 3) The fittings corrode if used exposed outdoors.
> 4) They come loose.
>
> ==========================================
> >
> > Another question...I know it may not be ideal, but with a
> > generous loop, can I use this type of cable around a rotator
> > and avoid the losses from a set of connectors and the
> > flexible jumper? My total runs are about 40-60 ft and below
> > 1300 MHz. Alternatively, should I use the flexible version
> > of LMR400 instead?
> >
> > Thanks &73,
> > Gordon, w2TTT
> > 201.314.6964
> >
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|