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Author Topic: 2 antennas on one radio mobile  (Read 5194 times)
KJ4YZI
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Posts: 38




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« on: March 06, 2012, 04:34:38 AM »

i got a question for the smart ones out here, i have a kenwood d700 and a mirage 2 meter 160watt amp, lmr 400 and nmo hard mounted on roof to a mfj1432 antenna, 7/8wave on 2 meters, 3 over 5/8 wave on 440, highest gain i could find.

what im wondering is, how would i put two antennas on my one radio, maybe without a switch? i know a duplexer lets two outputs to one dual band antenna without switching, but im thinking someone told me that a 1/2 wave inner city will outperform a 7/8 wave in the city, and i just thought maybe a couple different antennas would give me the ability to talk maybe to very distant stations and locals as well. im always looking for more gain and distance, im a sucker for 1.0db even tho it doesnt make a difference, supposedly i have the highest gain antenna right now, and i do like the 2 meter monoband 8 footer, with 7.2db gain or whatever, it would be cool to switch to the 2 meter monobander when that repeater is starting to fade out, however i think i just answered my own wuestion, maybe i do need a switch....does anyone know the pinout of the plug on the back of the mirage amp so i could make an interface myself? should be real basic ....
 
any other help would be great, regarding antennas for different applications. i been doing this for a while now, trying to make the best out of my setup, but there is way more people that know more than i do. thanks
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K0BG
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« Reply #1 on: March 06, 2012, 08:39:42 AM »

Well, playing the Gain Game won't garner you as much as you might think. If you're operating via repeaters, the major consideration is the HAAT difference of the two. Typically, repeaters are mounted much higher than average terrain for very obvious reasons. When you use a gain antenna, most of the high angle gain is suppressed in favor of lower angle gain.

If you're trying to work simplex, a higher gain antenna might be advantageous, depending on what the other stations HAAT is, and what he's using for an antenna as well.

On FM you have to increase your ERP by at least a factor of 4 before you see any real difference in distance coverage. Even if you increase your ERP by 10 times, you don't get much more than a 25% improvement if that much. Again, terrain is the issue.

Lastly, trying to combine two same band antennas isn't easy, and probably would cause you more problems than it would solve.
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K1CJS
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« Reply #2 on: March 09, 2012, 08:18:06 AM »

In addition to what Alan said, there is the problem of power output.  If you have two antennas that are resonant on one frequency, the power output of the tranceiver will be split between the two antennas, effectively reducing the power you're transmitting with and possibly the range of your rig. 

Also, more than likely, you're going to have problems with the phasing of the two antennas to allow them to work together, and a directivity problem--since the two antennas together will work like a beam antenna.  Best you decide on one antenna and stick with it.  Even swapping one for the other when you want to change them isn't worth the extra effort.
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WX7G
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« Reply #3 on: March 09, 2012, 08:37:36 AM »

http://cometantenna.com/products.php?CatID=1&famID=6&childID=0

Comet makes several duplexers and they run about $60.
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W8JX
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« Reply #4 on: March 09, 2012, 08:49:09 AM »

In addition to what Alan said, there is the problem of power output.  If you have two antennas that are resonant on one frequency, the power output of the tranceiver will be split between the two antennas, effectively reducing the power you're transmitting with and possibly the range of your rig. 

I really like this one. Think about this. If you feed to resonant antennas while it would split power between them there would be no loss of ERP (possible gain if phased) because same total power is being radiated.

I used two antennas, one for VHF and one for UHF on mobile for many years. Nothing wrong with it. I actual used a total of 4 different ones. In town I used a through roof 1/4 wave on 2m and 440 via a duplexer (2m 1/4 wave is 3/4 wave on 440 and resonate). When traveling I would switch to using 1/4 wave for 2m and a mag mount collinear for 440. Sometimes I would switch to a mag mount 5/8 wave for 2m in "sticks" and on a few occasions even used a full sized collinear on 2m on a custom multi-magnet mount on roof. I had to be careful when using 5/8 in some areas because on tall height and especially when using 2m collinear so I used 1/4 wave a 440 collinear mag mount as default most of time.  Did get some impressive range out of 2m collinear traveling cross country at times and some funny looks with it sticking up off roof of SUV.
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AA4PB
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« Reply #5 on: March 09, 2012, 10:09:27 AM »

A diplexer typically routes VHF signals to a VHF antenna and UHF signals to a UHF antenna. It won't work for connecting two VHF antennas to a single VHF radio. For that it would be best to use a manual switch to select the antenna (one at a time) that you want to use.

If you connect two VHF antennas together then the total ERP will be the same as if you had only one antenna BUT you don't know where the combined antennas will put that radiation. The pattern will change depending on the antenna spacing and the feed line lengths. In addition, if you parallel two 50 Ohm antennas you will have a feed impedance of 25 Ohms which will give you an SWR of 2:1.

Two similar antennas can be properly phased and impedance matched to provide a directional pattern (which results in gain in that direction) but that is not normally very useful in a mobile environment where you never know in what direction the vehicle will be pointed.
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N6AJR
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« Reply #6 on: March 09, 2012, 10:12:07 AM »

Antennas, there are thousands of books on the subject.  for a repeater the hi gain vert is about as good as you can get.  For better results think about using a beam Yagi and some sort of antenna switch  and  you can get a multi band beam or a switch and several mono band antenna, or even stack beams for more  directionality.

so it depends on what you want but some research into different antenna systems in called for  here.  What you are talking about  is fairly basic.  see about joining a local and see how other folks in your area do it.  there are lots of ways to go here. have fun. By the way, you can leave the amp in line  to a dedicated 2 m antenna  with no problem  when off.
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N6AJR
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« Reply #7 on: March 09, 2012, 10:14:40 AM »

oops, I missed the part about mobile. but a switch will still work, as will a diplexer.  I havs a pickup with 5 radios in it and use 4 nmo mounts for the roof for 2m 440 220 mhz 900 mhz and hf. the 4 aqre mounted through the roof  and 3 antennas are mounted on the bed rails in back.
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K0BG
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« Reply #8 on: March 09, 2012, 12:55:42 PM »

While the effective ERP of two properly phased and identical antennas is the same, but not if you use disparate antennas (apparently what the poster wished to do). And, if you use two identical antennas, you might end up with null lobes in the direction you're wishing to communicate.

It is also important to point out, that placement is much more important than gain.
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