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Galileo
Posted on Friday, October 10, 2003 - 9:26 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

I have a radio Shack Mobile mag mount antenna...I made a pole standoff for the antenna, and plan on using it for my base...The mag mount attachs to a small steel plate, anyway I want to add radials to the plate for a groundplane effect..Any ideas...These antennas are on sale for $9.99 so I got a couple...Just want to tinker, Thanks, Tom Shaw KD5YQF
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ss8541
Posted on Friday, October 10, 2003 - 10:13 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

i have never done this with a mag mnt antenna. but this should work. from each corner of the sheet of metal (ASSuming that is square/rectangle) attach a 19" piece of metal rod or wire. you may have to trim this down to get the best reflected power/swr.

i have done something similiar to this on fiberglass firetrucks and it worked well. it was done with nmo type 1/4w vhf antennas mounted in the center of the cab(some fire dpts think that mirror mnts are for truckers). between the fiberglass roof and the roofs interior i made 20" wire radials all 90* to each other and trimmed them down for the least reflected power. i them duct tape them to the roof and replaced the roofs interior. some of the 1st ones i did was 2yrs ago and they are stil in service today with 100w motorola maratrac's. so i know that this will work for those types of antennas and installations, and hopefully will with a mag mnt even though the capacitive grounding through the magnet isn't as good as a true gnd.
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de
Posted on Friday, October 10, 2003 - 11:45 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Per the book, radials in theory should be slightly longer than a quarter wave length. Most books state 5% longer.

To determine a quarter wave length just divide 234 by the transmit frequency in Mhz to get the length in feet.

Take that answer and multiply it by 1.05 and there you go.
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ss8541
Posted on Saturday, October 11, 2003 - 9:34 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

de you are right for a by the book answer(some books anyway). the 234 you gave is a 1/4w when using a metal wire. free space 1/4w is 1.05 times longer or 246. so for grnd radials you are basically using the free space formula, which is still a 1/4w.

and if you do the math(the 246 formula) for 146mhz(the middle of the 2mtr band)you will get 19", the length i gave him. i have found that the ones i did usually tuned up best with the radials somewhere between the free space 1/4w length and the physical 1/4w length. so i do the math for free space, and then trim down from there. i haven't done a ton of these, but for the few that i have done, this is what i found.
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Moderator558
Posted on Saturday, October 11, 2003 - 10:08 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

WOW you just answered a problem I was having on a friends Pontiac with a plastic body we couldnt get it to work very good at all so I will be back later on today and let you know how it worked,,,,,,,,,,,its only a Pontiac Fiero but everyone wants a radio in their car!!!!!!!!!!

Thanks Richard
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Moderator558
Posted on Saturday, October 11, 2003 - 10:21 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Richard

one more question,,,I have the ICOM V8000 2 meter radio and we want to install that on our boat which is all fiberglass,,,the marine antenna has some funkey coax that I seen and has what seems to be a ground wire on it that connected to a ground plate which in turn connected to battery negative,,,,,,,,,,,,is this correct will this work with the V8000 or should I use something different ,,,i never did trace the wire to see if it actually connected to battery ground I am just ASSuming so,

What do you think would be best?
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ss8541
Posted on Saturday, October 11, 2003 - 5:47 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

558,

i would go with one of the nmo type groundless 1/2 or 5/8 wave antennas by antenex, maxrad, cushcraft, antenna specialist, etc. most of these are tunable from 144-174 and will handle 100-200w. the gain is usually about 2-2.5db. your v8000 at 75w would work well with these(just don't dump the 75w into your maha amp!!!, you know what i mean).

the only reason i had to do what i did in the past was because the antenna's were mounted on the roof of fire trucks. their bays were not very high, so they did not want to go with a 5/8w antenna because of the height. they were afraid that they would get caught in the electrical bay door openers and rip them off. it had happened in the past. so they had to go with the 1/4w antennas which we all know need a ground plane. since there was not one, i had to make one. the install guys used some metal tape that was supposed to create a gnd plane. it worked about as well as the fiberglass roof. it was ripped off and we then went this route.

you can use a 'marine' antenna but many of these are only tunable from 156-162mhz. they sometimes come with a folding mast(a plus). but, they aren't very cheap. i have used the shakespear version for game wardens/wildlife officers and they worked well.

if you find a 'marine' antenna, pay attention to its operating range since you plan to use it for 2 meters.
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Barracuda
Posted on Saturday, October 11, 2003 - 6:58 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

By the way, I believe that the antennas that Galileo mentioned he got from RS are 5/8 wave. At least the ones I got from them were (also $9.99).
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Mr_Rf
Posted on Sunday, October 12, 2003 - 12:19 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

558,

You can buy a sticky-backed copper tape that can be applied on the inside of the fiberglass/plastic vehicle's body part to provide a groundplane effect. Professional two-way shops usually have this stuff. Also, if you can't find the copper tape I have used the sticky-backed alumunum tape that is used to seal heating/cooling ventilation ducts in your home. It seems to work equally as well and cost 1/4 the price.
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ss8541
Posted on Sunday, October 12, 2003 - 11:28 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

mr. rf, if you reread my post above you will see that we tried that tape. the type of tape described by the manufacture to provide a gnd plane. like i said, after covering the whole roof, it worked about as well as a gnd plane as the fiberglass roof itself did.

with the gnd radials made of wire, we were able to obtain an swr of 1.5 or so on all vehicles.
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ss8541
Posted on Sunday, October 12, 2003 - 12:42 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

i was being a little critical here. let me say that the tape did work for me the times i used it. it may work for someone else in a different installation. i was just trying to give something that i knew from experience would work.
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Mr_Rf
Posted on Sunday, October 12, 2003 - 8:23 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Hey, missed the above detail. Pardon me!
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Galileo
Posted on Sunday, October 12, 2003 - 9:50 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Hey guys, thanks for all the answers to my question....One more thing should the radials be straight out or at an angle???? Yes, this is a 5/8th's antenna...Man what a deal at $9.99...They work excelent in mobile app's....I can hardly wait to see what kind of results I will get for my base..Thanks again, Tom Shaw,KD5YQF
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ss8541
Posted on Tuesday, October 14, 2003 - 12:20 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

since i'm not sure how this will work out with a mag mnt, and 5/8 wave, to begin with, i'd say try both ways.