Does coiling LMR400 affect the SWR of the Antenna System

[Anthony Rizzolo]

I had to replace my feedline because it got cut by the guys doing my lawn this weekend.  It usto be RG8X.

I figured as part of the replacement I would upgrade this section of coax with LMR400 (2 50′ sections with a barrel connector).

Unfortunately this left me with a lot of antenna wire at the Antenna.  I took an SWR reading before I did anything and it was about [email protected] and 14.125 MHZ.

I finished up and noticed a lot of LMR400 on the ground so I put it into a coil to make things easier.  When I took another reading the SWR had shot up to over 2.2.

Note that I might be able to get away with a 25′ length of LMR400 but before I go and replace the 50′ length with a shorter one, or better yet just invest in some crimp connectors and tools to make the lengths have no extra, I wanted to know that this increase in SWR is being caused by the loop and the technical part of me might want to know why.

Thanks.

Anthony Rizzolo/KC1DXL

 

[Fred Kemmerer]

This is pretty odd. I’d suggest you try testing your LMR-400 into a dummy load to confirm that the cable does not have a problem. Also, check the barrel connector and the connection at your antenna to see if anything is loose or not making a good contact.

Did you create the coil that caused the high SWR near one of the connectors? If so, you might have a bad connector at that point…

[Hamilton]

Loosely coiling coax should not change the match unless it is part of the antenna design, or damage to it or the connectors.  Is there a balun or current choke at the antenna?

[Anthony Rizzolo]

Thanks Fred and Hamilton.

Yes, the coil was near one of the connectors so perhaps that is the problem.

Yes, there is a balun on the Antenna.

I will try as you say to try doing the SWR into a dummy load with the coil but might have to wait till the weekend till I can drop the Antenna again.

I’m pretty sure that the connections are all solid but will verify that as well.

Tony

 

[Hamilton]

Just thought. Wire antennas can be very sensitive to height. Is there a chance your antenna is being weighed down cause it to be lower. That would raise SWR. Connect your analyzer and pull down a bit on the feed and see what happens.

We are also assuming by coiling the inside diameter of the coil is 8″ minimum and preferrably 12″ min. Wound too tight SWR will change.

Last, the old cable may have had a problem and the antenna was tuned around it.

[Anthony Rizzolo]

Yes, Hamilton I think you might be onto something.  Tonight I came back and wound the excess cable into a loose into about an 8″ minimum.  Also the way I had done it over the weekend the coil was off the ground so putting pressure on the Connector on the Balun.  Tonight I put the cable on the ground and and and left the cable so there was no pressure and I am getting SWR of 1.2-1.6 at 40m and 1.5-1.4 80m…20m is the same as 80m so it looks good.

I might go and get a shorter length (25′) of LMR400 and see if that makes things better (after I measure the wire on the ground) as in less cable on the ground.

Thanks to both you and Fred for your quick help on this.

Tony

 

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