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Building an antenna

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Mattkuhar

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hey guys I have a GR1225 repeater that im getting ready to setup (just need to get the duplexer re-tuned) but i was wondering, is it ok to build an antenna for a repeater? or should I just bite the bullet and buy one?

It will be for GMRS use, and power out of the repeater will be 45w, into about 15-20' of coax to the antenna. Looking to have something with a good amount of gain, while also being affordable too. Whatever antenna I end up using/building will be using N connectors to match the repeater.



Thanks, Matt
KC1CQO/WQUS933
 

mmckenna

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Nothing wrong with that.
If it's designed, built and tested and it checks out on your frequencies, then it won't hurt anything.

Where you might want to consider other options would be how well it's going to last at your repeater site. Since many repeater sites involve high towers, mountain tops, buildings, etc. they can take quite a beating. Designing your antenna with that in mind would be fine, but you may discover it's easier/cheaper to buy a suitable antenna.

But, yeah, sure it'll work.
 

Mattkuhar

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Nothing wrong with that.
If it's designed, built and tested and it checks out on your frequencies, then it won't hurt anything.

Where you might want to consider other options would be how well it's going to last at your repeater site. Since many repeater sites involve high towers, mountain tops, buildings, etc. they can take quite a beating. Designing your antenna with that in mind would be fine, but you may discover it's easier/cheaper to buy a suitable antenna.

But, yeah, sure it'll work.
my site is actually on a school, about 800' ASL, with not a lot of wind (60mph gusts are not frequent). I was thinking a piece of 2-3" PVC pipe as the shell of it, cut to whatever length i wind up needing.

Is it ok to make an antenna at the full wavelength, and/or 2 or 3 wavelengths long? or will that cause issues? This will be the first antenna for transmitting that I have made, so im trying to cover my butt, and get help along the way :)
 

jonwienke

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Multiple-wavelength antennas generally have undesirable radiation patterns.

That is why most antennas are either 1/4-wave or 5/8-wave ground planes, or 1/2-wave dipoles.
 

prcguy

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If you have reasonable soldering skills and something to test the antenna with its not bad. BTW, the gain on that antenna is more like 6dB and you would need about 16 elements or twice the length to get the 9dB they claim.

Another great performing antenna that is within the relm a garage project would be a 4-element exposed dipole array. It can be made from Home Depot parts but you would still need an antenna analyzer to make sure the element and coax lengths are on track. I designed an easy to make VHF version which is available here on RR but I don't have any plans for a UHF version. There should be something available on the Internet somewhere.
prcguy



might be building one of these: Build A 9dB, 70cm Collinear Antenna

Thoughts?
 

Mattkuhar

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I am perfectly content with 6dB of gain :)

What would I need (other than a radio) for testing the antenna? I dont have access to an antenna analyzer unfortunately, Is there an easier way to check it out?
 

Rred

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Matt-
Do note that a coaxial collinear design like the one you refer to has dimensions marked to the single millimeter. So you're construction skills had better the good enough to beat that spec, or the antenna won't perform properly. And that's working on squirming coaxial cables, where length is often not so easy to fix. Plus, when coaxial cable is vertical it sags, compresses, changes length and characteristics again. And if you use tie wraps (as they do) those compress the insulation in the cable--changing characteristics further. So some commercial coaxial collinears put the finished antenna in a "tube" and then fill it with an adhesive wax or other material, to pot the antenna and keep it from shifting.
Good for experimenting, not so good for a first-time user who doesn't want to keep checking the antenna every couple of months. FWIW.
 

Skypilot007

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If you have reasonable soldering skills and something to test the antenna with its not bad. BTW, the gain on that antenna is more like 6dB and you would need about 16 elements or twice the length to get the 9dB they claim.

Another great performing antenna that is within the relm a garage project would be a 4-element exposed dipole array. It can be made from Home Depot parts but you would still need an antenna analyzer to make sure the element and coax lengths are on track. I designed an easy to make VHF version which is available here on RR but I don't have any plans for a UHF version. There should be something available on the Internet somewhere.
prcguy
I've constructed this antenna in the UHF version and the collinear antenna as well, both for use with GMRS repeaters. Once I got the correct parts in place on the exposed dipole array it is the better antenna of the two. Your results may vary. I eventually bought a high dollar decibel products antenna because of the environment where the antenna was installed (roof top at 65ft on an ocean front house). I got tired of repairing the home brewed antenna. I use it at home now and its fine.
 

Mattkuhar

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Matt-
Do note that a coaxial collinear design like the one you refer to has dimensions marked to the single millimeter. So you're construction skills had better the good enough to beat that spec, or the antenna won't perform properly. And that's working on squirming coaxial cables, where length is often not so easy to fix. Plus, when coaxial cable is vertical it sags, compresses, changes length and characteristics again. And if you use tie wraps (as they do) those compress the insulation in the cable--changing characteristics further. So some commercial coaxial collinears put the finished antenna in a "tube" and then fill it with an adhesive wax or other material, to pot the antenna and keep it from shifting.
Good for experimenting, not so good for a first-time user who doesn't want to keep checking the antenna every couple of months. FWIW.

Hmm, funny you should mention that, I was actually just talking with my co-worker about that as well, I think im gonna go with something simpler for now, single element with some radials as the GP. seems easier to construct, and should give me decent range
 

Mattkuhar

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I've constructed this antenna in the UHF version and the collinear antenna as well, both for use with GMRS repeaters. Once I got the correct parts in place on the exposed dipole array it is the better antenna of the two. Your results may vary. I eventually bought a high dollar decibel products antenna because of the environment where the antenna was installed (roof top at 65ft on an ocean front house). I got tired of repairing the home brewed antenna. I use it at home now and its fine.
I was actually looking at one of the antennas from them as a matter of fact, but dont currently have the ~$80 for it right now, that is the plan in a few months though
 

jim202

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Matt-
Do note that a coaxial collinear design like the one you refer to has dimensions marked to the single millimeter. So you're construction skills had better the good enough to beat that spec, or the antenna won't perform properly. And that's working on squirming coaxial cables, where length is often not so easy to fix. Plus, when coaxial cable is vertical it sags, compresses, changes length and characteristics again. And if you use tie wraps (as they do) those compress the insulation in the cable--changing characteristics further. So some commercial coaxial collinears put the finished antenna in a "tube" and then fill it with an adhesive wax or other material, to pot the antenna and keep it from shifting.
Good for experimenting, not so good for a first-time user who doesn't want to keep checking the antenna every couple of months. FWIW.
The last company I know of that sealed their antennas inside the fiberglass pole went out of business more than 30 years ago. They used bees wax and just heated it up and poured it in with the antenna upside down.

The actual lengths of each element are not that critical. Laying them on a workbench makes it easy to construct and solder together.

I would suggest not using the white PVC pipe as it is not UV protected and will not last long out in the sun. You might try using the gray PVC pipe. You can ask at the hardware store and see what they suggest. Some PVC pipe has been known to cause a reaction to cause the antenna inside it to no longer be functional on the frequency you tuned it for.

Just a few comments from someone that like playing around with home built antennas.
 

amphibian

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If you build it they will come.... It's who shows up afterwards that makes the difference in a great antenna or a bad one.....

So, if you are going to place it on a building that's not of your own please do yourself a favor (and the other occupants of that building that may be using other RF gear) and borrow, rent or purchase a spectrum/comm analyzer, a real RF meter (like a Bird) and make sure all your test are run by someone of professional RF background.

I can't tell you how many times over the life of my working career in the RF field the amount of home built antennas I found causing interference with local in-house radios after someone in the building thought they would put up a ham or commercial antenna they built.

I have no problem with people building their own stuff, heck I do it all the time... I in fact, I encourage where I can but I also encourage it with the understanding that it be checked out before putting into final operation. Until you have been on my side of the spectrum and had to go hunt down a interference problem you just can't appreciate what I am talking about. But once I or FCC shows you will appreciate it afterwards...so save yourself and others a lot of grief and at least get someone with extensive antenna building knowledge to help you with it. Most communication shop techs like sharing what they know, if not then try your local ham club and see if they have anyone with at least three years experience and a extra class license to help you.

I wish you well in your build.....
 

Rred

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Are you saying that "home built antennas" caused interference?
Or are you confusing that with "a new radio transmitter" caused the interference, and would have done so regardless of who made that new antenna what was put up along with it?

I can't see how a home-built antenna, as opposed to a store-bought one, would *cause* interference.

Please, do enlighten.
 

kb2ztx

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So I like building stuff and think its a great hobby. With that said I would suggest picking up a commercial antenna online for the repeater especially if its going at a school. You want a professional install and good gear. I am at many sites and know of guys getting tossed out of sites for shody installs, and homebrew stuff. Alot of Ham clubs over the years have had to come up to standards because of this also.

You can get a good antenna for a reasonable price.

My first antenna was this mount - BASE GROUND PLANE KIT NMO to N CONNECTOR VERSION | eBay for under $30.00. I installed a 3DB gain antenna to start, then when i had a few dollars I picked up the 5db gain mobile antenna (NMO Mount) to install. I have since upgraded to a DB404 I got from a local radio shop. There are hundreds of options online.

I actually just installed this antenna on my GR1225 on one of my sites for $99.00 on sale.
TRAM 1485 UHF 450-470MHZ 5dBd BASE REPEATER ANTENNA MOTOROLA GR500 GR300 GR1225 | eBay

So far good results
 

Rred

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PIM: Just when you thought it was safe to go back into the water.(G)

Thanks, N5. Never heard of that one before. I guess to some extent that means that every "junction" in an antenna, even clean stainless or others, has to be considered as a possible active element.
 
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