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Author Topic: 2 meter SWR and feed line length  (Read 7068 times)
KG7QYJ
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Posts: 26




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« on: March 28, 2016, 06:05:15 AM »

I can get an SWR of 1.1 - 1.2 on a 2 meter antenna at the source. Problem is the radio I want to use is in my shack. I have a 75ft length of RG-213 as the antenna feed line. If I put the SWR meter at the other end of the 75ft line in my shack, the best SWR I can get is 3.0-3.1.

I understand the impact feed line length has on this issue but I was wondering is there were any tricks to better results?

Do any of you guys have a 2 meter antenna further away?

Thank you!
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N1UK
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Posts: 2129




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« Reply #1 on: March 28, 2016, 06:43:16 AM »

Sounds like a cable problem. Is the shield making a good connection to the PL-259.

I would try another cable and see.  You should not be seeing this with a good setup. You have a cable/connection problem.


Mark N1UK
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AA4PB
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Posts: 14496




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« Reply #2 on: March 28, 2016, 07:11:09 AM »

As you move back away from the antenna feed point the SWR reading should improve due to the loss in the coax. Your readings indicate a problem with the coax or connections.
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Bob  AA4PB
Garrisonville, VA
KG7QYJ
Member

Posts: 26




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« Reply #3 on: March 28, 2016, 08:43:44 AM »

After reading your replies, I tested something and figured out my problem.

When my antenna line comes into the shack I have it on a Daiwa switch (2 separate antenna lines). I then have the line go into an MFJ-969 antenna tuner and out to another Daiwa switch that I switch between the HF / 2 meter ports on the back of a FT-991.

I assumed the COAX BYPASS setting on the antenna tuner acted like a clear through line for when I want to use the 2 meter antenna. Apparently it is not?

As soon as I connected the incoming antenna line straight to the SWR meter bypassing all that....I got a very nice 1.3 SWR.

Can I get some thoughts on that?  Is it the switches? Or the 969 bypass issues I mentioned?

Thank you!
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K6AER
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Posts: 4918




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« Reply #4 on: March 28, 2016, 08:46:03 AM »

Coax cable has loss. With enough cable and no antenna you would have a great VSWR just because of cable loss. With the antenna being a good match you must have a bad connection at the connector end. The connectors could have a bad solder joint or the cable might be damaged due to water, crushing or some other anomaly. 
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WB6BYU
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Posts: 17483




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« Reply #5 on: March 28, 2016, 08:46:56 AM »

If the SWR is already 1.1 - 1.2 at the antenna, adding any feedline (assuming it is the correct
impedance, the meter is properly calibrated, etc.) should only lower that reading due
to coax losses.  That's why the others are suggesting there is a problem with your coax.

As an example, using 75' of RG-213, an SWR of 1.2 : 1 at the antenna would give 1.12 : 1
in the shack.

Or, an SWR of 7 : 1 at the antenna would give an SWR of 3 : 1 in the shack.  If you're reading
3 : 1 in the shack, something is clearly amiss!


But this might also depend on the antenna you are using:  if, for example, you are testing
a mobile antenna that is not properly grounded, the coax braid becomes a significant part
of the antenna.  In that case, changing the coax length actually changes the antenna, so
one would expect the SWR at the antenna to change.  An SWR of 7 : 1 is still a pretty
big shift, however, so start by checking the connectors on your coax and make sure they
are properly soldered (especially the shields - that's where I find most of my problems.)
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AF6AU
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Posts: 66




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« Reply #6 on: March 28, 2016, 08:55:39 AM »

I would do several tests... First use a alligator clip type wire jumper and short out 1 end of the coax, test at the other with a good FET Ohm meter. Wiggle the ends. The total DC resistance for Good RG213 should not be more than a couple ohms. Take off the jumper. Check again, you should read infinity.

Next Check using the Ohm meter with the dummy load installed. The Ohm meter should read under 55 ohms.

Next use LOW POWER RF, and a SWR meter. With Dummy load on the cable, the SWR should be perfect or near perfect 1:1. Test with the dummy load OFF (key up only for a second), and the open ended cable SWR reading should read full scale, or close to it.

Coax that is lossy will not peg the meter. Really absorptive coax may read a lower SWR value, and thus the coax is a dummy load in itself. This is a easy and simple test. With Dummy load, a near 1:1. Open ended, the higher the SWR the better.

Long lengths of Coax on VHF are many wavelengths long, and as the other guys mention, the SWR improves (usually) as the cable's absorption (loss) tends to not only attenuate signal but the standing waves as well. (Best explanation I know anyway.) A good antenna (as you verified), and good coax, you should have a great overall SWR.

I bought a 100 footer of Foam core Rg8 that the machine ran short of foaming plastic, and there was ~9 inches where the foam necked down. Externally the cable looked great. The dummyload test showed it was bad. I found it when coiling up the cable, that internally thinned area bent awful easy, then by pinching the cable lightly with pliers it was (for lack of better terms) "Squishy". Back to HRO it went.

In short(er) runs, length can come into play. I did a mobile install using a Larsen NMO mount kit in the center of the roof. Clean metal, nice grounding, perfection. The coax was about 3 feet too long, so I cut, and installed a new PL end. I even did the 50 ohm dummy tests to good results.  What were really great matching antennas (a Comet SBB5 (1.2 to 1) and a Larsen gain 2 meter 1.1 to 1) went to pot (3 to 1 at best). I replaced the cable back to the full length, SWR was perfect again, and I made a few 1-1/2 foot jumpers as Club give-aways.

Good luck..
AF6AU
So dig out the dummy load.
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N1UK
Member

Posts: 2129




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« Reply #7 on: March 28, 2016, 09:05:16 AM »

Quote
I assumed the COAX BYPASS setting on the antenna tuner acted like a clear through line for when I want to use the 2 meter antenna. Apparently it is not?

Read the spec if they publish them but most tuners are only good up to 30 MHz. Keep your 2m signal out of the tuner and run it directly to the rig or use a coax switch designed for vhf such as the Alpha Delta mechanical coaxial switch series.  

I believe MFJ make a special 2m tuner but your average HF tuner and switching is only good up to 30 MHz and maybe 50 MHz for a few of them.

Is your Daiwa switched speced for 2m ?


Mark N1UK
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AA4PB
Member

Posts: 14496




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« Reply #8 on: March 28, 2016, 09:09:25 AM »

After reading your replies, I tested something and figured out my problem.

When my antenna line comes into the shack I have it on a Daiwa switch (2 separate antenna lines). I then have the line go into an MFJ-969 antenna tuner and out to another Daiwa switch that I switch between the HF / 2 meter ports on the back of a FT-991.

I assumed the COAX BYPASS setting on the antenna tuner acted like a clear through line for when I want to use the 2 meter antenna. Apparently it is not?

As soon as I connected the incoming antenna line straight to the SWR meter bypassing all that....I got a very nice 1.3 SWR.

Can I get some thoughts on that?  Is it the switches? Or the 969 bypass issues I mentioned?

Thank you!

I expect that the tuner (in bypass) is NOT rated to handle 2M and above. Often, the bypass setting routes the signal through various PC board runs and relays that don't present a good 50 Ohm impedance at VHF and higher frequencies.
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Bob  AA4PB
Garrisonville, VA
KG7QYJ
Member

Posts: 26




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« Reply #9 on: March 28, 2016, 09:21:41 AM »

Thank you for the VERY informative responses and the time to construct them. I've definitely learned something.

The RG-213 is literally only a week old.

I don't know if the green-ish Daiwa switch is rated for 2M? I thought they were a pretty universal mechanical switch?

I will bypass the 969 completely as well.

Thanks again!



Man, I definitely know where to come for the experts.  Wink
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N1UK
Member

Posts: 2129




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« Reply #10 on: March 28, 2016, 10:00:48 AM »

Quote
I don't know if the green-ish Daiwa switch is rated for 2M? I thought they were a pretty universal mechanical switch?


No a lot of them aren't rated above 30 MHz.

These are

http://www.dxengineering.com/parts/alf-delta-4b

Rated  through 500 MHz.0.10 dB loss to 30 MHz, less than 0.5 dB through 450 MHz.

and they make a 2 position one in the same case as well.


Enjoy...it's all part of the learning process and i am glad that you were able to get some help. here.

73 Mark N1UK
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G3RZP
Member

Posts: 167




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« Reply #11 on: March 28, 2016, 12:30:00 PM »

An easy way to get a good idea about the cable is to leave the fare end open and not connected. Then measure the SWR. It should be high......by using the formulae from measuring the SWR, you can figure the return loss (how far down the reflected power is) and thus the cable loss.
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