Dual truck antenna considerations

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KC9LQV

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I'm posting this to the railfan section because the setup I am installing is geared almost exclusively for railfan monitoring. All other considerations are secondary. That, and I'd like to see if I can't filter out some of the inevitable off-topic arguments that end up running most radio-related threads into the ground.

I'm installing two IC-2720H rigs and two Larsen 5/8 NMO mount whips in my '03 Chevy S-10 pickup. It's an extended cab, so there's a pretty generous area to work with. I've got the headliner and trim out of my truck, have the routing figured out, the hole saw and the whips are on order.

My big question is, what's the best arrangement of two antennas. Side by side? Front to back? How much "clear area" does each antenna need before they start interfering with each other? Both whips will be cut for the railroad band.

Eventually, I will likely install a third rig for strictly 2-meter use, with a dedicated antenna mounted on the rear of the truck bed. But in the meantime, should the need to transmit arise, am I going to damage the non-transmitting rig by having two antennas on the truck roof? This is why I'm not using a single antenna and a splitter. (I also realize that a 161 tuned antenna isn't ideal for 2-meter, but I've used my old mag-mount Larsen 5/8 to get in to the occasional repeater on low power and it's been acceptable.)

I'd like to get thoughts and feedback on the best approach for this install, but hopefully keep it on the practical side.

Thanks in advance ...
 

FFPM571

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If it is an extened cab, There should be no problem to mount them as far apart as possible . As a rule When I mount 2 antennas of the same band I use a minimum of a 1/4 wave separation. So a vhf 1/4 wave antenna is roughly 18 in and a chevy truck is like 46in wide I would find the center of 23 in and then measure 18 in out from center. Then you would have a full 1/2 wave between them. If you were to add a 3rd antenna you could add it to the front center of the cab in a triangle just do it roughly 18in forward of the center line. Since you are using the 2 antennas for mostly recieve its not an issue really but should you transmit you dont want to overload the front end of the other.
 
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N_Jay

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As far apart as practical.

More than a 1/4 wavelength is so/so.
More than 1/2 wavelength is better.
More than a whole wavelength is even better
 

KC9LQV

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How much open ground plane does a 5/8 whip ideally need? I'm assuming that I need to balance between getting the antennas as far apart as practical, but not so close to the edge of the truck that the ground plane becomes lopsided.

On the rare occasion that I would transmit with one rig, I realize that the RX on the other unit will be shot, and would just turn it off. But is there any risk of actually causing physical damage to the second receiver even if it's turned off?

Thanks for the fast replies!
 
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N_Jay

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a 1/4 wave of ground plan is probably all you need.

The exact coupling of two antennas mounted in the near field is very hard to predict with enough accuracy to make a meaningful guess.

Additionally, the point at which a receiver will be damaged by an excessively strong signal is also a guess in most cases.
 

stevelton

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A friend of mine had his Yaesu FT7800 dual band radio hooked up to his base antenna at his house. He also had a new to him VHF maratrac 100 watt radio he had hooked up to another antenna on his tower. The antennas had about 15 foot vertical seperation. He left the 7800 on one day while he was transmitting on the 100w maratrac. It blew up the 7800's receiver. He had to send it to yeasu to have it fixed.
As for the railroad antennas.
With the 2720 being a ham radio, why not cut the antennas for the 2m band to begin with? There wont be any difference in how well it will receive, weather its cut for 161 or 146.
Also, the 2720 is a dual band 146/446, isnt it?
The railroad uses UHF for the EOTs. a 2m 5/8 isnt going to work very well on UHF.
Just my thought. Good luck
Steven
 

KC9LQV

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Well, that's certainly scary. I don't want to risk damaging my equipment. If there's a consensus that says "don't TX with a dual rooftop set", then I won't. Checking into the occasional net isn't worth killing a rig.

Should I just bail on the idea entirely, and stick with a single antenna in the sweet spot and just split the input in the truck? Sure, the splitter will give me a minor performance hit ... but is it worth fifty bucks to avoid it if I still can't transmit safely?

Regarding the EOTs, yeah, the 2720 will pick them up. However, in my experience, listening to EOTs is fun but not all that useful. Their range is so short that by the time I pick up an EOT, I've usually been listening to the train call signals for 10 minutes. It's not a big consideration for me.
 

FFPM571

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There are plenty of public safety users who run multiple radios in the same band most at higher powers that dont have issues. Run an antenna for each radio. Your perfomance will be much better than using any kind of duplexer or splitter. Also a 18in VHF 1/4 wave is a 3/4 wave UHF so your 440 performance would be good with each radio.
 

byndhlptom

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Dual antennas

If I understand your original post, you are using the Icoms for RX only with an upgrade to a 2mtr tx./rx later.

If I was doing this, I'd install the two rx RR antennas on the front fenders (1/4 wave) and save the roof for the 2mtr antenna. You will get some isolation from the tx, and as someone mentioned prev, 1/4 wave whips are pretty dependable on rx. I have that setup for my scanners & have had zero problems. If you use NMO mounts, you can always move stuff around.....

my current setup is 11mtr/6mtr/2mtr/440 antennas on the roof, three scanner antennas on front fenders/cowl (92 explorer)

good luck with your install, have fun

tom
 

FFPM571

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Im not a fan of front fender antennas the ground plane sucks and the chances of picking up engine noise increased
 

prcguy

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A 5/8 Larsen is fairly narrow band and in my experience one cut for 2m will hardly pick up 162Mhz weather channels in my area. I can swap my 2m Larsen for a 1/4 wave whip and weather channels go from almost unusable to full quieting.

Two similar VHF whips 16-18in apart will give one main lobe and a bunch of messy side lobes that are noticeably degraded from a single whip. Two VHF whips 32-36in apart will give a clover leaf pattern with 4 degraded nulls. Pick your poison or keep them very far apart.

Also, the VHF 5/8 Larsen sucks on UHF and although a 1/4 wave VHF whip presents a good match at 3X the frequency (UHF) the radiation pattern has a null at the horizon and points up at about 45deg.
prcguy
 
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KC9LQV

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Thanks for all the feedback fellows. I knew I could get some good discussion going in this subforum.

If my idea (2 whips on the roof/2 receivers) is fundamentally flawed, I'm more than willing to bail on it. I'm not committed to it ... at least until I start drilling holes.

I don't want to do anything that's going to degrade the kit's performance at 161. So by all means, feel free to suggest an alternate arrangement.

My goal is to run the two ICOMs for railroad monitoring at maximum efficiency, with the option of occasionally transmitting on 2 meters, even if it means adding a third rig and leaving it off 98% of the time.
 
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N_Jay

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Does the little bit of gain from the 5/8 make that much difference?

Do you usually monitor sitting still or driving?

What not a taller ground plane-less antenna on a separate mount (like a marine antenna)
You could get some gain, and a good bit of additional height. (probably more important than gain.)
 

KC9LQV

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Does the little bit of gain from the 5/8 make that much difference?
I don't really know. Perhaps? All I know for sure is the 5/8 mag mount I have now is the best performing RR antenna I've ever used. Even using my lowly HT, it pulls in amazingly well. When it came time to consider installing a permanent antenna, my first instinct was simply to go with what's worked in the past.

Do you usually monitor sitting still or driving?
I would say about half and half.

What not a taller ground plane-less antenna on a separate mount (like a marine antenna)
You could get some gain, and a good bit of additional height. (probably more important than gain.)
As I mentioned, I'm open to other arrangements and possibilities. I've also toyed with the idea of a folding mast attached to the inside of the bed of the pickup with a base antenna (like a Traintenna vertical) attached to it that could be folded up and down quickly when I'm parked for a long time. Sometimes I do enjoy just finding a high hilltop overlooking the tracks and just listening to the scanners, and that would be great for that ... but obviously not practical while moving.
 

N8IAA

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Scott, my suggestion is a dual band antenna. They are usually 1/2 wave on 2m, and coliinear on 440Mhz. Since you have an extended cab, do you have an extra door. Not sure if that is the case with the Rangers. Had a 150 with a third door. That is where I had an antenna for the scanner, and had my transmit antenna on the hood. Used low power for transmit, really good repeater sites here in GA. That way, you have separation. One of my best performing dual band antennas is a Larsen 1/4 on 2m and a 1/2 on 440Mhz. It is a shorty antenna that is spring loaded. I have also had luck with a Laird 150/450Mhz commercial dual band.
HTH,
Larry
 

KC9LQV

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It's true the 2720 is a multi-band radio, but I set both "sides" to monitor the railroad band. UHF reception just isn't an issue for me.

Since that's the case, is there still an advantage to using a dual-band antenna?
 
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