Antenna Building Troubleshooting

[Sailor]

Mods move this if needed to the proper forum. No comm forum and since there is a Comm sticky here this is where I put it.

I built a role up j-pole out of 450 ohm ladder line per this link A NEW and IMPROVED roll-up J-Pole Antenna by Chris McBride, KB7QXQ

Feeding it with about 30 feet of RG-8x I cut, crimped and soldered with pl-259's. I cut the long and short elements down till I got to about 1:1 SWR per my MFJ-259B analyzer.

I went back after covering all exposed wire with liquid tape and tested and now the SWR is 8.4? I put a 6 foot purchased coax lead on and its 1:1. Both 30 foot lenghths I have are at about 8:4.

So my question is what/how do I test or narrow down my problem? I fully admit being a noob on the engineering side of ham, but I am learning.

 

[woowoo2]

I am not into ham radio, but I do know a little about RF.

It seems to me that you need to tune the antenna with the down lead attached and all insulating materials in place.
Only then will you know the true reactance of the transmission line.

You are trying to match 450 Ohm (Antenna) to 50 Ohm (PL259)

There will always be a little SWR ratio, you are just trying to minimize it.

 

[ranger2k]

Forgive me if I'm mis-reading this....

I think you are saying:

-that with the antenna and your 2-30ft pieces of coax, your SWR is high.
-that with the antenna and a 6ft pre-made piece of coax, SWR is fine.

Is that correct?

If that is the case, I would put the 259B into test mode and test those 30ft lengths of coax for issues OR just hang a dummy load on one end of the coax and use the 259 to sweep the spectrum.

 

[Sailor]

Correct with the 6 ft piece all is well. But I initially cut and test the ant. with my 30 ft piece of coax and had it at 1:1.

I just got the analyzer in last week. So I put the analyzer in Coax Loss mode to test it?

I have a dummy load once on I scan for the freq with the lowest swr?

I thought I was good to go on the cable, I tested before with 1.0 swr, may be a short? I will test for continuity on the center/ground.

 

[ranger2k]

Sounds like a possible issue in the longer piece of coax. I would do just as you stated, check to see if there is a short between center and shell of coax connector.

Another check is to connect the dummy load to one end and the 259 to the other, then just start sweeping the frequencies. You should see little to no SWR across all bands.

Don't let low SWR fool you. It's not always an indication that things are well in a antenna system. A dummy load should always give you 1:1 VSWR - and you can't really work many folks on a dummy load

 

[Sailor]

Ok I will report back once I get some time in with this new analyzer.

 

[woowoo2]

The cable should 50 / 75 Ohm no matter how long it is....

 

[45-70]

Sounds like the feedline is radiating. Try a 1:4 balun to decouple the feedline from the antenna and see what you get.

73

N9MB

 

[yotewacker]

Ladder line is a balanced coax. RG sytle cable is an unbalanced coax cable. Either way it will need to be turned to an transmitter. N9QKN

 

[Sailor]

I tried an air coil no luck.

The ladder line is not being used as a feed line. Each side is being used as a leg of a dipole, although not conventionally as one would think of a dipole.

I think I figured it out though. Put my multimeter on the shield and center of each end. Solid 1's but occasionally flashing to zeros. There is a short somewhere. WTF? it is brand new cable and putting on PL-259j's is not rocket science.

The only thing I can think of is that since this RG-8x is thin, I applied to much heat when soldering the center conductor, and cooked the insulation. This would explain it.
I remade them and only put enough solder in just to flow in and keep the center conductor from moving.

Hooked it up and it reads 1:1 swr on my freq with an x of 2, so its nearly resonant where I want also. I will clean up the connections and test it further.

Now I need to find some RG-8x crimp sleeves so I reuse my cut off connectors!

Thanks for the help.