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Author Topic: How important is a groundplane below a 2M 5/8 ante  (Read 2037 times)
N2MDB
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Posts: 14




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« on: September 19, 2001, 03:56:04 PM »

I know having a good ground plane (such as the roof) is important for a 1/4 wave 2M antenna, but how about a 5/8 wave antenna? Would putting a 5/8 wave antenna on my trunk lid have an adverse affect compared to the roof? I know when I put a 1/4 wave 2M antenna on my trunk lid, I get a lot more "fluttering" in the signal compared to the roof, which I presume it due to not having a good ground.
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KC8AXJ
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Posts: 303




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« Reply #1 on: September 20, 2001, 12:05:11 AM »

As you have said with your 1/4 wave, there will be a difference( in the pattern forward of the vehicle)
vs. your element having a nice 360 degree ground plane
that a good size rooftop can give your antenna to "work with". BUT, being the height of the element(5/8 wave) gets it up there where the center of the element (where most of the radiation strength comes from) can work with your roof(and over it), it should give a decent pattern forward.
I know my terminology may be a little off but as far as discribing the antenna functioning in all directions, BUT (another but) with VHF, were're not talking working DX or E.M.E. here. I think someone or a repeater  15-20
miles ahead(forward) of you wouldn't notice a hole lot of difference in the ol' S meter either way.
The BEST thing you have done for your VHF radio is bought a 5/8 wave antenna You'll love it !!!!
73
Steve kc8axj@arrl.net
 
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KB9YNB
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« Reply #2 on: October 09, 2001, 02:24:41 PM »

As I understand it, 5/8 wave antennas are ground independent by design, and therefore don't REQUIRE a ground plane in order to function well.  I have my 5/8 wave 2m antennna on my Jeep with a home-made bumper mount, and am able (on a good day) to raise a repeater about 60 mi away.  This antenna has NO groundplane.

That's not to say that a ground plane won't affect your antenna's performance, it will.

Hope this helps.
KB9YNB
Ryan
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WA4DOU
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Posts: 436




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« Reply #3 on: October 11, 2001, 11:38:39 AM »

The 5/8th wavelength ground plane IS absolutely dependent on a good ground plane under it. It is the 1/2 wave vertical that is not dependent on a ground plane under it for the condition of resonance, only. Even then it is still desirable.
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K8KAS
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Posts: 570




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« Reply #4 on: October 15, 2001, 07:43:06 AM »

We have tested 1/4 wl vrs 5/8 wl mobil antennas a number of times on a commerical antenna test range and found the 5/8 wl antenna to be very sensitive to location. I would take a 1/4 wl antenna on the roof over a 5/8wl placed anywhere lower on the body. The numbers have proved this out. The 5/8 wl is very  Ground Dependent. To get ANY gain out of the 5/8 wl you need a good sized (van roof) ground plane with the antenna in the middle of it. What happens when the 5/8 wl is placed on the lower parts of the auto body the antenna becomes directional or has major holes in the pattern. In a perfect situation the 5/8 wl only provided about 1.5 db improvement, can you see a 1.5 db gain, I can't. I
like a roof mounted 1/4 wl. The bumper is not an option for placement of a VHF antenna. You could bury it and do better.
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KB9YNB
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« Reply #5 on: October 16, 2001, 02:15:55 PM »

WA4DOU:  you're right.  I checked my docs, and the antenna I have on the Jeep right now IS a 1/2 wave Diamond brand antenna.  My previous was a 5/8 wave (on 2m) dualband antenna.(also Diamond brand)

I had it (the dualbander) mounted to a trunklip mount on my Nissan Altima, and it still got out fine with minimal ground plane.  When I sold the car, it also worked fine with the radio on my desk and the trunklip mount resting on a steam radiator cover next to the desk. (2 sq ft ground plane)  I was still able to hit repeaters about 40 miles away, with acceptable signal reports. Before the flamethrowers spark up, remember I'm just relating MY EXPERIENCE.  You might have different results.

Bottom line, it's going to depend not only on the electrical length of your antenna, but the quality of the design, the loss of the feed system and connector, and lots of other factors on your vehicle.

As for the comment on bumper mounts... it's my only option.  The fiberglass roof on my CJ7 doesn't provide any ground plane anyway, and when it's off, it can't be used as an effective antenna mount! (unless I had about 100 km of 9913!!:^) by mounting the antenna to the bumper behind the Jeep, I also protect it from low tree branches and other obstacles on the trail, and from parking garages and drive thrus etc.


I was loath to cut a hole in the roof of my new car because I knew I wasn't going to keep the car.  I chose the trunk lip mount.  A bumper mount would have worked too.  Bottom line, it's still going to work better than  a glass-mount, and way better than anything inside the car (that I can think of!)  Best of all, you can take it with you if you get a new car, and you won't hurt the resale value.  Also, A 5/8 wave bumper-mount antenna gets out INFINITELY better than NO ANTENNA AT ALL.
If you're looking for a 2m antenna only, go with the half wave and put it wherever you can.  I recommend Diamond Antennas and Comet mounts.  I have 2 of each, and the have all taken a lickin' and kept on tickin.  Your mileage may vary.  The antennas and mounts have survived cold winters and hot summers in Southern Indiana, and are NOT garage kept.

Closing:  Sorry for the misinformation on the 5/8 wave antennas (although, the manual for mine says that it is designed to work independent of ground, take that as you will)  Your license gives you the right to operate in a safe and non-interfering way.  Take advantage of that and EXPERIMENT, find a combination that works right for YOU.

73, KB9YNB
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N2MDB
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Posts: 14




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« Reply #6 on: October 29, 2001, 04:25:18 PM »

Here is what I did...I bought the Radio Shack 5/8 wave mag mount (figured if it was poor antenna I would just return it). It's a LONG antenna, so I put on the trunk lid. Works MUCH better than a 1/4 wave on the roof. And as a bonus, I (with some work since the guts of the connectors are reversed from usual) built a simple 1/4 wave whip (just a 19" piece of stiff wire) to use for 'round town driving (or for low profile use). I just unscrew the 5/8 whip and coil and screw on my 1/4 whip.

I didn't try it on the roof to compare it to the trunk, but I'm very happy with the trunk mount of the 5/8 for highway use (less than 1.2:1 SWR). And it's acceptable for 440 use too (less than 2.0:1 on the SWR across the 440 band) even though it's not marketed as dual band.

Thanks for all the comments.
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