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Author Topic: Some thoughts on antenna building.....  (Read 805 times)

Offline N8XEE

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Some thoughts on antenna building.....
« on: March 21, 2015, 11:35:45 UTC »
I recently posted (on another forum) about deciding on which antenna to build/erect.  My current antenna is a G5RV Junior at about 40', fed with RG-58.  I was considering replacing that with either a coax-fed parallel dipole for 40 and 20, or a single 40m dipole fed with twinlead.  Either antenna would have covered my main bands of operation - 40, 20, and 15. I was also going to install a pulley at the apex to make it easier to swap/maintain my antenna.   I ended up with the parallel dipole.  Here are my results:

1. I suck at antenna building.
2. Murphy hates me.
3. A pulley really isn't necessary. In fact, it's a PITA. At one point, I let go of the rope. Yes, you guessed it - the antenna came crashing down, and the rope went flying up...... all the way up. I spent 20 minutes using the "whiplash" method trying to get the rope back down. I finally realized that I could just lower the pulley. No one ever said I was the sharpest crayon in the box.
4. You need more in your toolbox than WD-40 and a roll of duct tape. You need a hammer and a screwdriver, but mostly a hammer.
5. Parallel (fan) dipoles are absolutely miserable to work with.
6. If you think you need 2 people, you probably need 4. I was alone.........
7. You WILL leave your favorite coffee mug out in the rain.

After all of that, I'm still running the G5RV.....  Cheers.
Current callsign - KC3RN

Offline GM0LVI

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Re: Some thoughts on antenna building.....
« Reply #1 on: March 22, 2015, 20:47:02 UTC »
Kevin, antenna building is also character building  ;D

I'm midway through construction of a trapless tri-band portable GP. Tuning and pruning is going to be a lengthy and frustrating process, but IF I can get it working the blood sweat and tears will be worth it!
Dave

Offline N8XEE

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Re: Some thoughts on antenna building.....
« Reply #2 on: March 26, 2015, 13:35:14 UTC »
Dave, you're absolutely right!  I built a BUNCH of character that day.....  ;D

Another thing that I found is that you just cannot overlook the importance of the feedline.  Please keep that in mind when you're tuning and pruning.

I had been feeding my antenna with 100 feet (about 30 meters) of ancient RG-58.  I used that simply because that's what I had when I originally put the antenna up.  I recently replaced that with 50 feet of Belden 9913, and 50 feet of new RG-8X.  Once again, I used what I had, although I'd much rather have a single piece of the 9913.  Regardless, the difference was very noticeable.  On average, my received signals went up at least 1 "S" unit, and the noise went down considerably.  I also appear to be getting better signal reports from stations I've worked, all QRP.

Because of this experience, I've adopted the opinion that the feedline is at least as important as the antenna itself.

Good luck with the pruning!
Current callsign - KC3RN

Offline GM0LVI

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Re: Some thoughts on antenna building.....
« Reply #3 on: March 26, 2015, 18:39:39 UTC »
Yes Kevin, the feedline is a critical component between the radio and antenna. The antenna can only put out what has been fed in when transmitting and the received signal can be significantly attenuated due to feedline loss.
I've completed work on my 3 band portable GP. Tuning took just a couple of hours and all bands look good..
12m - VSWR is 1.3:1 Z is 50 ohms
17m - VSWR is 1.2:1 Z is 48 ohms
20m - VSWR is 1.1:1 Z is 54 ohms

The FG-01 Mk2 analyser proved invaluable when pruning. I'm using RG174 to feed it as I have to keep weight to a minimum.



Dave

Offline N8XEE

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Re: Some thoughts on antenna building.....
« Reply #4 on: March 27, 2015, 11:00:03 UTC »
Dave, nice work!!!  I was really wishing for an antenna analyzer when I was working on the dipoles!!! But what really opened my eyes was actually measuring the feedline loss.  I didn't realize how simple it was, but very revealing.

First, I measured the actual output of my radio, by connecting it directly to a dummy load, with a watt meter between the radio and dummy load.  I was getting a solid 4 watts.  Then I inserted the feedline.  On 20 meters, the output (at the end of the RG-58, was just under 2 watts.  Ouch!!!  I was losing half my power up the feedline!!!!  That's JUST feedline loss - there was no antenna connected, only the dummy load.......

Next, I substituted the 9913/RG-8X combination, and was reading just about 3 watts.  Still losing a watt, but seeing a 50% increase in efficiency.  I'd call that significant.

Prior to this little "experiment", I was of the opinion that the single most important factor in wire antenna performance was height.  I see now that I was mistaken.  Each factor is equally important - yes, height, but also efficiency (SWR), and feedline loss.  It's a 3 part formula.

Now, being a ham, I'm never really content with my antennas.  I'm not wondering if I could get another incremental improvement by replacing the G5 with a doublet, fed with open wire line.........

The quest continues. ;)
Current callsign - KC3RN