[Elecraft] SWR Readings:Differences

Wed Sep 3 09:44:47 EDT 2014

Here is a small, non-inclusive list of things that can be bad with a coax
run to the house. I have personally seen all of them at some time or place.
Some big names, too.

1) At either or both ends, the shield was not soldered to the PL259 shell,
and the connection has dwindled to a small percentage of the shield due to
gradual destruction of the fine wire touching points.

1a) Ditto for the center conductor to the hollow center conductor pin of
the PL259. Amazing how many PL259's are not soldered in a hurry to 'hear
how it works". I've done it myself and forgotten I didn't. For years.

2) "Balun" is cheep junk, or burned up/melted/shorted
turns/cracked/destroyed core, now junk.

3) Coax on tower is not high quality, and does not have it's weight
supported. The stretch has changed the separation of shield and center
conductor, and the characteristic impedance has shifted, introducing an
unwanted transformation into the equation.

3a) Coax is very old, but still "looks OK" even though it isn't. Much coax
material undergoes very slow (even decades) chemical changes which can
change characteristics. Wide frequency and TDR scans of coax into opens,
shorts, checked 50 ohm and 200 ohm terminations are only way to check for
still-OK-ness.

4) Water has invaded the coax, from a nick, tear or critter bite in the
jacket, or non/poorly sealed coax connector, and capillary action has
wicked along it's entire length. This can be hundreds of feet in the worst
of cases. The loss-added coax does not necessarily stay at 50 ohms Z zero.
I have seen coax shields green (copper oxide) their entire length.
Interestingly the practical outcomes of this extra loss was initially most
often blamed on the transceiver.

5) Coax has been wrapped around a pipe by rotator torque. See 3)

6) Coax has had something heavy dropped on it. See 3)

7) Coax shield was only ground path for induced current for a close
lightning strike. (Usually a direct strike smokes coax beyond any
confusion.)

8) PL259 shell was not pliers-tightened and has worked loose, gradually
producing burned points of connection.

9) Long coax run was laid tight in summer heat, and stretched in the
winter. See 3)

10) Constant flexing of coax finally breaks the center conductor. After
that, "connection" is strange.

11) Unsupported aerial coax over-weighted by ice and stretched, pulled out
of connectors, see 8).

12) Coax is innocent and it's really the antenna.

13) Operator in fact does not know how to operate the instrument or how to
interpret readings.

14) Short lengths of coax and "other boxes" are ignored, and those in fact
contain the problem.

15) Measuring instrument has been damaged or was defective from the
factory.

15a) Measuring instrument is cheep junk.

16) Operator was "told" what the trouble was by a "trusted source" and is
having a lot of trouble thinking outside of the "trusted box" when the
trusted source was in fact in error for this instance.

17) Dummy load used to calibrate/provide 50 J zero comparison or reference
termination is not 50 J zero for any number of reasons, including the likes
of several seconds of QRO on a 2 watts worth of 50 ohms. Didn't turn black
on the outside, but the resistor innards were already toast. Didn't *look*
burned, so must be fine, right?

18) Dummy load while accurate at DC has significant reactive components at
RF.

19) Coax was not 50 ohms from the get-go (try 56). But it looks good so it
must be good. And the seller had a nice looking web page and the best price.

20) I need to wrap this up, but I know I'm not remembering something really
juicy, which will come to mind after I hit the send button.

A group of Olde Pharte hams sitting around a table in a Lunche Jointe, came
up with a napkin version of this that had over 40 items on it. The napkins
were used for the inevitable outcome of ribs, and so was lost to posterity.
One of these included a male F connector that had not had its threads
grooved into the shell. This ignominious occurrence is not mentioned above,
because it could not ever be part of a "working" setup. But it was amazing
how many other things were blamed for "not fitting", including calls to an
equipment maker about female chassis connectors, before the lack of threads
was noticed.

Good luck to all and 73,

Guy K2AV

On Wed, Sep 3, 2014 at 12:18 AM, Vic, K2VCO <k2vco.vic at gmail.com> wrote:

> If your line is lossless (it isn't) you would expect the same SWR
> readings anywhere along the line. With practical lines that have some
> loss, the SWR should be LOWER farther away from the antenna. You are
> getting the opposite result.
>
> One cause of erroneous SWR readings is RF flowing on the outside of the
> coax. If your rotary dipole doesn't have a balun, this could be the cause.
>
> It's also possible that you have a bad connector or bad piece of coax
> between the tower and the K3.
>
>
> On 9/3/14 3:06 AM, pastormg2 at verizon.net wrote:
>
>> Good Evening, This is Mark Griffin, KB3Z and I have some questions
>> regarding the SWR readings I get at my tower versus what I get on my
>> K3. I will give the SWR readings that I got at my tower for a 40
>> meter rotatable dipole at 55 feet.
>>
>> Tower:
>>
>> 7000  2.2 7025  1.8 7050  1.5 7075  1.3 7100  1.0 7125  1.1 7150
>> 1.3 7175  1.6 7200  2.0 7225  2.2
>>
>> K3 Readings:
>>
>> 7000  3.5
>>
>> 7025  3.2
>>
>> 7050  2.9
>>
>> 7075  2.6
>>
>> 7100  2.4
>>
>> 7125  2.3
>>
>> 7150  2.4
>>
>> 7175  2.5
>>
>> 7200  2.6 7225  2.9
>>
>> What would cause such a big difference. The cable run from my antenna
>> switch on the tower to my K3 is only an additional 75 feet. I am
>> using RG-213 cable. Is there anyway that I can test the SWR reading
>> that my K3 is giving me?
>>
>> Mark Griffin, KB3Z
>>
>
> --
> Vic, K2VCO/4X6GP
> Rehovot, Israel
> http://www.qsl.net/k2vco/
>
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