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Posted: 10/2/2011 9:12:46 AM EST
Thought that some one here could help me with this.

1) how far apart do 2m antenna's need to be so that they dont effect each other? 1 wave length?

2) on a 1/4 wave ground plane antenna. I see some use the ground radials at 90* and some have them at 45* downward. Does this change anything? if so which is better?
Link Posted: 10/2/2011 10:00:01 AM EST
[Last Edit: 10/2/2011 10:02:08 AM EST by Gamma762]
Originally Posted By gcw:
Thought that some one here could help me with this.

Arfcom can answer any question.

1) how far apart do 2m antenna's need to be so that they dont effect each other? 1 wave length?

Define "don't effect each other", ie, a more through description of what you are trying to do and how critical the application is.

2) on a 1/4 wave ground plane antenna. I see some use the ground radials at 90* and some have them at 45* downward. Does this change anything? if so which is better?

It does change something, a couple of things really.

A 1/4 wave monopole over a ground plane has a typical feedpoint impedance of something like 35 ohms. This gives you a slight mismatch to the typical 50 ohm coaxial cable and transmitter, resulting in a small but easily manageable SWR (like 1.2:1)

Depressing the radials down at an angle does two things: increases the feedpoint impedance, to allow an essentially perfect match to 50 ohms, and also tends to increase the antenna signal (gain) performance at the horizon slightly by lowering the main lobe of the signal pattern.
Link Posted: 10/2/2011 11:23:27 AM EST
Ok thanks for the info on the radials. that was easy enough.

As for the the distance part. What Im wanting to do is build a repeater system in a box. A while ago I asked about building a cheap repeater and got some good answers. I have decided to go with a simplex repeater. My plan is to use two 2m only radios in an old pelican case I have, with a battery and power supply. One set up for the simplex repeater and the other one as a digipeater.

I would like to make a spreader bar for the 2 antennas, the mount the bar on a tripod. Im just trying to find out how far apart the two need to be from each other. so that I can use the simplex repeater and have packet traffic at the same time with out causing any problems.
Link Posted: 10/2/2011 2:46:51 PM EST
You can separate the two antennas enough to avoid one drastically altering the pattern of the other. Your biggest problem is going to be each transmitter desensing the other receiver. To minimize that you will want vertical separation.
Link Posted: 10/2/2011 3:42:21 PM EST
I'm IN on this for more info. Wanting to do the same thing, but I'd don't want to get to far ahead of myself seeing how I haven't even recieved my callsign yet.
Link Posted: 10/2/2011 3:45:32 PM EST
Yep, you have two different issues at work here - antenna interaction and radio interaction.

You would need high gain antennas and too much vertical separation to be suitable for portable use if you try to do two simultaneous systems on the same band in the same location - unless you use a duplexer.

The two practical options that would work would be to have separate setups for the voice and digipeater systems so that you can locate them some distance apart (adjacent hilltops or something like that), and just using different bands for each system. One system on 2m and the other on 70cm and you could run both into a single dual band antenna and be set.
Link Posted: 10/2/2011 4:34:24 PM EST
Originally Posted By Gamma762:
and just using different bands for each system. One system on 2m and the other on 70cm and you could run both into a single dual band antenna and be set.[/quote]

I like this Idea. It could make the second part of the project easier (adding HF)

So here is what Im thinking
-VHF digipeter for aprs or packet. using something like a yeasu 1800 or kenwood 271a. with a 1/4 wave ground plane.
-UHF with simplex repeater box. not sure on radio. and same 1/4 wave antenna.

Im really wanting to do the antenna as a 1/4 wave because I have an idea to make the elements with arrow shafts like my arrow hand held yagi. this way they will be able to break down and fit in my case. I would also like to stick with 2 antnnas just for more versitility.
Link Posted: 10/2/2011 4:45:27 PM EST
One of my winter projects is to mount a pair of 2m beam antennas on one mast. One would be horizontal polarization and the other would be vertical polarization. Horizontal would be on top and the vertical on bottom. At any time only one of these antennas would be connected to a radio. So all I was concerned with was how close could I be and not have the inactive antenna interact with the active antenna and vice versa.

Figured the guy who built the antennas should know so I shot him an email with the above question. A day later I got a response.

"As long as the closest element to the other antenna is no closer than 8 inches you are good to go."

Units are ELK 2M/440L5.

As for how close when both are active, don't have that knowledge.
Link Posted: 10/2/2011 8:42:43 PM EST
[Last Edit: 10/2/2011 8:45:43 PM EST by Gamma762]
Originally Posted By HankEllis:

For horizontal/vertical polarization you could put both yagis on one boom for that matter. I think it's Cushcraft that makes (made) an antenna just for that, with the two sets of elements perpendicular and a switching box to allow you to switch between the two so you only have one feedline.

There are also crossed yagi antennas set up for circular polarization for satellite or EME that could be split into separate antennas. Or just use them in circular and accept the 3dB loss.

Then again with the Cushcraft split antenna, or presumably if you homebrewed an antenna, you could tune each of them for their respective parts of the band, since horizontal you really only need 144.0 to about 144.3, and for general purpose use on vertical probably center tuned at 146.5 or so.
Link Posted: 10/3/2011 4:42:56 AM EST
Originally Posted By Gamma762:
Originally Posted By HankEllis:

For horizontal/vertical polarization you could put both yagis on one boom for that matter. I think it's Cushcraft that makes (made) an antenna just for that, with the two sets of elements perpendicular and a switching box to allow you to switch between the two so you only have one feedline.

There are also crossed yagi antennas set up for circular polarization for satellite or EME that could be split into separate antennas. Or just use them in circular and accept the 3dB loss.
Thought about doing the circular polarization thing just to have a compact package. That 3db loss killed the deal for me. 3db is cutting the TX output in half. I'm already working with low antennas (19ft) in an attic and can't afford to give anything away. Yes there are the crossed yagi designs but those that I found with a quick Google search were way over the top for my use and wouldn't fit in the attic anyway. The just over $100 Elk gives a lot of bang for the buck

Then again with the Cushcraft split antenna, or presumably if you homebrewed an antenna, you could tune each of them for their respective parts of the band, since horizontal you really only need 144.0 to about 144.3, and for general purpose use on vertical probably center tuned at 146.5 or so.
"Normally" (what is normal anyway?) 2m SSB is done horizontal at 144.1 to 144.3. 144.0 to 144.1 is reserved for CW and digital. BUT ... locally we have guys who have rigs that can do 2m SSB but don't have a horizontal antenna. So having the capability to quickly switch from horizontal to vertical, switch from FM to SSB, and to swing the beam around to catch the other party is a big plus. Sounds chaotic but it really isn't.

There's probably 25 of us locals that are semi-serious about 2m and pushing what can be done with what we call "real world" equipment. Yeah a 100' tower and a giga-element beam is cool but with a DC to daylight rig such as the TS-2000 or the FT-897, a 'reasonable' beam, and a knowledge of propagation you can do amazing things. We have a 2m simplex net on 146.52 FM that has had checkins from 300 miles away. We're just really getting some traction on the 2m SSB net and even 50w to a vertical makes FM look so ... primitive. The only time I kerchunk a repeater now is during my weekly club net. Now that I've been shanghaied into a 2m digital experiment we'll see how that will pan out.
Link Posted: 10/3/2011 8:21:10 AM EST
If you're talking about attic-bound antennas, and for nets and the like, why not omnidirectional antennas? I've had really good results from a "big wheel" horizontal omni, and a stacked pair of them which could probably be shoehorned into most attics would be even better. Put a pair of verticals at opposite ends of the attic, one tuned for 144.2 and the other to 146.5 and you'd have all the bases covered.
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