I would chop the brass rod off at 20 inches and trim a little more until your SWR comes down to 1.5:1 or less (stop at 19 in if you don't see lower SWR)>for 2M & 70 cM.
2+ wave is a very poor choice for UHF . Those 20 ft long, fiberglass pole, commercial, UHF antennas have multiple coaxial dipoles in a line phased for low angle radiation ! More complex than they look.
62 inches is a good 6M length but you also need 62 inch radials.
Not much 6M FM used in Detroit area either.
A little local 6M SSB but everyone local stays quiet until band openings and contests come along.
73.....JD FISTS #3853,cc 455
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There is a small problem with modeling your antenna exactly. The radials are insufficient for decoupling the common mode current from the feedline, and that means the pattern is a large function of the feedline length and how it is terminated. With the antenna at 10 ft, the pattern is probably somewhere between the red and blue traces shown in the first figure below. At 10 ft elevation the low angle gain is probably between -2.9 and -0.5 dBi. That is about 4 to 6 dB less than you can get with a 1/4 wave ground plane at 10 ft height.
The second plot shows a comparison with probably the best gain you will get with this antenna and the 2 meter ground plane I built, both at 10 ft height. The antenna I built was a traditional ground plane except it had two sets of 4 sloping radials, separated by 19 inches. That was done to make it immune to feedline length.
Jerry, K4SAV
Edit:
Playing with the model a little more, I found an even worse case of feedline length than the one in the plot below. The gain with that feedline length was -3.5 dBi.
Yes, this isn't a very good antenna design for either band (2m or 70cm), really. It has "negative gain" at the horizon, as Jerry indicated, and the horizon is usually where you'd want to have whatever gain is possible.
There's a common misconception among "antenna newbies" that "if 1/4-wave is good, then longer must be better!" and that theory only holds true when you make the antenna differently -- not if you just make it longer. SWR and performance aren't related at all.
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There's a common misconception among "antenna newbies" that "if 1/4-wave is good, then longer must be better!" and that theory only holds true when you make the antenna differently -- not if you just make it longer. SWR and performance aren't related at all.
Actually, I wasn't going for the idea that longer was better, more that I'd seen the relationship between a 1/4wave on 2 meters being a good 3/4wave on 70cm, and wanting to try the 3/4wave on 2 meters.
I'd been hopeful about 70cm, but that didn't pan out. As it is, I tried the antenna again on 70cm with the SWR/ Watt meter, this time using a 3ft section of 8X between the antenna and the meter, and connecting through an adapter the radio to the meter, SWR came in around 2.5 to 1 if not worse, so the original lengths of 8X that had been used for testing the first time were far longer than they should have been, considering the losses in the UHF band.
This was a good experiment that didn't cost me very much in the bits and pieces it took to put it together (I think I might have $20 in materials put into this antenna). As to time, I think I spent 6 hours to put it together, most of it between getting the materials, and being distracted with other things happening around me, so no big losses, just learning.
To all that've responded, I appreciate the help in figuring things out(mostly that it wouldn't actually work all that great).
Originally Posted by WB2WIK
