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Author Topic: Directional Coax  (Read 10461 times)
TANAKASAN
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Posts: 933




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« on: September 26, 2013, 07:04:26 AM »

OK, we've all had a good laugh at the audio crowd with their oxygen free cables and gold plated mains connectors but now I have a problem and I'm stuck.

Four element VHF beam connected using a 12m length of RG213 and a PL259 at each end to an SWR bridge. The SWR reads 1.1:1. Disconnect the length of RG213, reverse it and reconnect and the SWR reads 1.3:1. Reverse it again and we're back to 1.1:1.

We've proved that we're not going crazy by marking the cable ends. Also the RG213 isn't made by Monster (R).

How can we have directional coax? The connectors are sound, the coax has been tested (along with the connectors) an doesn't show any issues and the SWR bridge shows 1:1 at the operating frequency with a dummy load connected.

Tanakasan
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AF6WL
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Posts: 126




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« Reply #1 on: September 26, 2013, 07:22:45 AM »

The cable has at least a dB of loss.
One of your connectors has a slightly worse match than the other.
This mismatch is reduced by the cable loss.
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K8GU
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Posts: 716


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« Reply #2 on: September 26, 2013, 08:01:30 AM »

The cable has at least a dB of loss.
One of your connectors has a slightly worse match than the other.
This mismatch is reduced by the cable loss.

That's what I would suspect as well.
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AF6WL
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Posts: 126




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« Reply #3 on: September 26, 2013, 08:55:33 AM »

There will also a be phase effect - the observed match will change with cable length/frequency.
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K8AXW
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Posts: 3599




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« Reply #4 on: September 26, 2013, 09:01:09 AM »

TANAKASAN:  OK, now I think I've heard it all and as soon as I get to the end of the Internet I can peacefully die.

If it was anyone else but you, I would love to say something like, "Any dumbass knows that you connect coax up in the correct direction!"

I think I'll go out and pull down my coax and reverse it.  Maybe that will help my SWR.   Roll Eyes
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WB6BYU
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Posts: 12983




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« Reply #5 on: September 26, 2013, 09:44:16 AM »

So, for example, if one connector had a shunt resistance of 170 ohms, that in shunt
with the 50 ohm coax would cause an SWR of about 1.3.  That's what the SWR meter
would read if the resistive connector were at the meter end, but if it were at the
far end the resulting SWR would be reduced by the coax losses.

But an SWR of 1.3 : 1 gets reduced only to 1.23 : 1 due to the 1dB loss of the coax
at 144 MHz.  So that, by itself, can't account for the whole shift (assuming that the
SWR meter has sufficient resolution and accuracy to detect such a difference.)


There are a couple other possibilities that depend on the electrical length of the coax.
For example, if there was a bit of shunt or series reactance at one connector, then
it might cancel the load reactance at one end, and add to it at the other end because
the phase of the impedance had shifted along the length of the coax.

I had a similar problem, but the SWR difference was much greater and the coax length
was only a few meters.  It turned out that I had a crimp plug on one end and a soldered
PL-259 on the other.  The SO-238 socket on the SWR meter had had a few too many
plugs with too much solder crammed into it over the years, so the center conductor
at the outside of the jack was sprung out and good contact only occurred as the
plug was inserted further.  This worked fine with a standard PL-259, but not with a
crimp-on connector where the tip was thinner. So one particular combination of plug
and socket made poor contact, but when the cable was reversed it made good contact
at both ends.  (I cut off and re-crimped several plugs before discovering the problem
was in the socket instead.)
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TANAKASAN
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Posts: 933




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« Reply #6 on: September 28, 2013, 02:43:16 AM »

Well, after all of the posts I decided to cut off the plugs at both ends and solder on new (as in fresh out of the individual plastic bag) PL259's. The antenna now measures 1.1:1 no matter which way the cable is connected.

But what happened to the plugs?

Well, I soldered a SMD 49.9 ohm resistor to the end of each coax stub then plugged them into a VNA. Plug #1 was an almost flat fifty ohms from 1 MHz to 150 MHz but plug #2 varied up and down from 45 ohms to 58 ohms depending on the frequency. It LOOKED identical but something internal (maybe the center insulator) has gone very wrong.

Tanakasan
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M6GOM
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Posts: 863




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« Reply #7 on: September 28, 2013, 07:13:23 AM »

The impedance of the connector is reliant on the ratio of the outside diameter of the inner to the inside diameter of the outer. Doesn't take much to alter the impedance 5 Ohms or so.
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G8HQP
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Posts: 119




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« Reply #8 on: September 28, 2013, 07:54:43 AM »

Has the coax been stretched at one end? Manufacturing defect? Water got in? A plug is not long enough to have much effect below UHF/microwave frequencies.
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K4SAV
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Posts: 1786




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« Reply #9 on: September 28, 2013, 08:39:09 AM »

This was a great question.  Congratulations to the guys who figured it out.

PL259's aren't 50 ohm impedance, but the mismatch is such a short length that it makes no difference at HF.  When you get to UHF, the length starts becoming significant, and the reason you often see N connectors used.  Your measurement verified that one PL259 measured 50 ohms up to 150 MHz.  The fact that the other one varied says that it has some additional resistance was added, either in series or maybe between the wires, or both.

Jerry, K4SAV
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K8AXW
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Posts: 3599




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« Reply #10 on: September 28, 2013, 09:16:50 AM »

GOM: Thank you for the headache! 
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KD0REQ
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Posts: 838




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« Reply #11 on: September 28, 2013, 11:10:22 AM »

my vote is for a little corrosion at the solder joint.

are you going to start selling coax cable makeups now with an arrow printed on the jacket?  $30 a foot seems reasonable for Radiophools.
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G3RZP
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Posts: 4328




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« Reply #12 on: September 28, 2013, 01:13:47 PM »

C'mon  REQ,

That's far too cheap!
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TANAKASAN
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Posts: 933




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« Reply #13 on: September 29, 2013, 12:25:49 AM »

See above comment regarding Monster (R) cables. An audio fool and their money are soon parted.

Tanakasan
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W1JKA
Member

Posts: 1616




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« Reply #14 on: September 29, 2013, 05:24:11 AM »

   As a low end  qrp  ham I have been following this thread with interest as I'm always trying to improve my current home brew antennas and refuse to pay $30.00 for an arrow marked coax feed line, so I end for ended my current feed line as suggested with inconclusive results. I don't own a commercial antenna analyzer, instead I rely on my ears and what is between them. Since this seems to be a potential major coax issue centered on the connectors that until now I was completely unaware of I decided to take it one step further and am now thinking about my soldered connections on the ends of each of my 30m dipole radiator wires and will take them down and end for end them also as soon as it stops raining. If I notice any signal improvement I will mark them appropriately. Thanks for the heads up.
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