Question for you antenna gurus.....

WK9F · Post by **WK9F** » Tue Nov 11, 2003 9:20 pm

Hi,

Here's one that has stumped the locals - maybe someone here has a solution.

My problem is this.....My HF antenna just doesn't work. I am using a commercially made G5RV Jr. (40 some odd feet vs. 100 and some odd and only good down to 40 mtrs I believe). It is installed in an inverted V configuration with the apex approx. 20' AGL with the ends approx 10' AGL.
I figured that this is part of my problem, but I cannot change it, landlord won't approve. The apex is supported by aluminum TV masts. The antenna is mounted to a steel standoff about 24" from the mast. The 15' ladderline is folded back on itself so it doesn't lay on the roof (I know, another problem). The antenna is fed with Belden 9913 coax. There is approx. 115' of coax between the rig and the antenna. There is a gas discharge lightning protection device installed inline connected to a 8' ground rod.

The antenna recieves very well considering the above. However, the SWR on TX varies between 10:1 and 7:1. I am not able to tune it with my MFJ-949E (bought new and verified correct operation at FD) on any band. Tuner has little to no effect.

I was told to install a dummy load at the antenna end to see if the coax is bad. SWR with the dummy load was 1.1:1 so we concluded that the coax is good (it was also bought new) and not shorted. This is not the only antenna I have tried. I have tried monoband dipoles that work at other locations with the same results.

Any ideas?

73 de Jerry

Nand · Post by **Nand** » Wed Nov 12, 2003 4:14 am

AB9EO wrote: The 15' ladderline is folded back on itself so it doesn't lay on the roof (I know, another problem).

This seems to be a big part of the problem since it is part of the matching network

Nand.

radioconsult · Post by **radioconsult** » Wed Nov 12, 2003 6:08 am

I suggest that you do a search on the G5RV and read up on the theory. The ladder line is actually part of the antenna system and should be laid out in a line and not folded. The true G5RV design does not use a balun at the junction of the coax and ladder line. On my G5RV I wound a RF choke consisting of 10 turns of RG-8x on a 1.5 dia piece of pvc. This reduces the amount of rf on the outside of the coax which may be the cause of the high VSWR. If you are landlord restriced, I would suggest a center fed dipole with as much antenna wire as possible fed with either 300 ohm twin lead or ladder line all the way back to your tuner and use the balanced output terminals on the tuner. This should provide some improvement in performance and may even work down to 75 meters.
Good luck.
Rich W5RDF

WK9F · Post by **WK9F** » Wed Nov 12, 2003 8:00 am

Hello,

In response to the ladder line being folded up on itself. This is what I was told by the locals: Once I did that, I essentially turned the antenna into a random length dipole. The reason I did this is two-fold: 1) so it doesn't lay on the roof and damage the shingles. 2) As stated before, the antenna is supported by an aluminum mast, letting this ladder line hang down would put it right up against the mast. I was told that this was worse than folding it back on itself.

I have also been told to use a random length dipole fed with ladder line or 300 ohm twin lead. This is not possible in this situation as the feedline is underground for 75 feet of it journey.

I will try the RF choke and see if that helps.

73 de Jerry

W4WTF · Post by **W4WTF** » Wed Nov 12, 2003 9:13 am

In your situation here is what I would do.

First, get that apex up higher if at all possible. Cut the ladder line on your G5RV at a point where it will almost reach the point where your coax comes above ground. Betwenn the coax and ladeer line, run a balun (if its 300 ohm line a 6:1, if 450 ohm a 7:1) and you have essentially made an off center fed dipole. Run as much of the ladder line as possible in your configuration, it does not have to go straight up and down.

Or, just run coax up to the top, eliminate the ladder line, and add a 4:1 balun at the junction. This may be easier for you, both because it eliminates the ladder line and because 4:1 baluns are stocked at most of the major retailers. If need for the lower bands, and some loading coils (a fellow on ebay sells some very nice premade ones cheap, or wind your own on PVC)

W4WTF · Post by **W4WTF** » Wed Nov 12, 2003 9:29 am

One more thought....

On every G5RV I have used, the feedline coax also radated to a degree, you can't run directly from coax to ladder line and expect it not to. The fact that you are running so much coax, and underground at that, coupled with this is probably the reson for your High SWR.

Switch it to something with a balun to keep anything from feeding back into your feedline. Given your description of the situation, I would lean more towards putting a 4:1 balun at the top of the mast and making it a coax fed off center dipole at this point, and possibly adding some loading coils if you have much difficulty on the lower bands. A 4:1 balun will run you under $20 from HRO or AES.