Vehicle Roof Antenna Spacing for Transceiver (1) and RX-only Scanner (2) [NEVER operated at the same time] ?

[chiwititsara]

I see a thread active today discussing multiple antennas and multiple radios. My situation is different in that one of my devices is receive-only.

So, this is either an odd situation or an easy answer, hence why I cannot find the same scenario via search:

In my Jeep, I regularly use a (1) VHF/UHF mobile transceiver. I also use (2) a handheld scanner sitting in my cupholder, often making use of the same NMO roof-mount antenna-to-coax-to-BNC connector that is primarily intended for transceiver use. I listen far more than I speak. And I NEVER have both devices turned on at the same time (since I only have one antenna) and do not need to use them at the same time.

For example, I'll be scanning via the roof-mount antenna and hear someone calling CQ on an amateur frequency. To switch to my (1) transceiver, I unplug the BNC-terminated NMO antenna-into-cabin coax and this same coax line-antenna is then plugged in to my transceiver so I can transmit.

This is *not* convenient/safe while driving down the highway (the disconnection process isn't bad, but trying to plug into the back of the mobile transceiver while driving is).

I do not use these two devices at the same time...EVER. If I hear CQ on the scanner, I move to the (1) transceiver... The (2) scanner is turned off and disconnected from the antenna. I don't have the attention span to listen to a scanner while talking on the radio anyway, and I rarely transmit.

A. I would like to mount a second antenna (perhaps a little wider-band, a little less gain/height) for the scanner so I avoid having to swap the coax back-and-forth while driving. This is a barely street-legal Jeep and I could not care less about cutting more holes and running more wires. I have tons of NMO mounts and antennas sitting around.

Assuming I do mount a second antenna...

B. Given the fact that these devices are NEVER used at the same time, is there ANY separation concern (distance from the existing antenna) regarding placement of a second, RX-only scanner antenna?

B(i). Note 1: This plan contemplates that If I am using my transceiver (on Antenna 1), the scanner will be at a minimum switched off, although since it is a handheld unit, I could easily disconnect the BNC-terminated cable from the scanner (if switching the scanner off is insufficient to avoid overloading a turned-off scanner through it's antenna, noting also that my mobile transceiver puts out no more than 20 watts on a good day).

B(ii). Note 2: If I switch back to using my scanner, the transceiver will no longer be transmitting and, unless I forgot to switch it off, not listening either. Life will be much easier when I no longer have to connect/disconnect the transceiver antenna coax to/from this mobile radio just to have an external scanner antenna.

C. The 'Obvious' Solution: By this point, you are probably thinking that the far easier solution is a manual switch that would cut-off one of the two devices from the (single, shared) antenna when the other device is using the antenna. However, these cost upwards of $50, introduce some amount of insertion loss (admittedly, not much), and most importantly, I already have a pile of NMO mounts, antennas, and no problem cutting more holes in my beater Jeep. The 'second-antenna' approach would be a 100% free project. And, with a second NMO mount, would open up later opportunities to buy a switch and use different types of antennas for different circumstances / bands.

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TL;DR - How much separation - if any - is needed between two vehicle-roof-mounted VHF/UHF antennas, each serving a separate device (one of which is an RX-only scanner) when the two devices (and thus the antennas) will NEVER be used at the same time.

As long as they are not nearly-touching each other, does separation even really matter in this scenario?

 

[Lauri-CoyoteFrostbite]

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Hi Chiwit

I am not quite sure what it is you want to accomplish, but when I saw you too had a Jeep I went out and took these photo's of my Wrangler-- I am making the assumption your's is a Wrangler too, and maybe my experiences can give you some ideas.

This first photo---
There are two antennas mounted on the Jeep's roof--- one is 19" tall, the other ~5"--- one's VHF, the other for UHF---
They are separated about 26 inches---- each connected to a separate Icom VHF and Alinco UHF transceivers.

Since the Jeep has a fiberglass roof, I had to fabricate ground planes for each--- those are-




-Two sheets of copper cemented to the roof's underside, approximating the area of quarter wave ground planes. They each have NMO connectors.
(That's a crow feather wedged under the coax feed on the UHF base btw )


(if you click on these photo's they will expand full format)

The SWR for each is a flat 1.5 to 1 on 140-150 MHz, 2 to 1, or so, between 420-470
Both radio's can operate pretty much at the same time--of course being aware of harmonics. They are fine with each other in such close proximity.
I appreciate that these are active radiators inches from heads, so its rare that the power levels exceed 20 watts, or so.

______________________________________

Questions ?


______________________________________

This is not the first time I have used artificial ground planes on similar vehicles. I can say that all of them worked superbly. and no radios--or brains--- have yet been fried.


Lauri



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[chiwititsara]

Hi Chiwit

I am not quite sure what it is you want to accomplish, but when I saw you too had a Jeep I went out and took these photo's of my Wrangler-- I am making the assumption your's is a Wrangler too, and maybe my experiences can give you some ideas.

Questions ?

Lauri

Thanks, Lauri. Very nice setup.

Copper is ideal for your purposes if a bit more expensive than steel. It is interesting that you have chosen rectangular sheets rather than circular. Would a quarter-wave-length radius, measured from each antenna base, fit within the area of each antenna's respective copper ground plane? I suspect this would be the most important consideration for ensuring omnidirectional radiation, even if there remains 'extra' metal outside the superimposed circle.

I appreciate your use of Laird NMO quarter-waves (right?). Having wasted money on more complicated designs promising 'gain', the simplicity, size and wide useable spectrum of a monoband quarter wave is my preference as well.

My vehicle is an Cherokee (XJ generation) though I am also a Wrangler fan. Of course, I do not have your issue of lack of a metallic roof.

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My own two-antenna objective is a little more complicated than yours in that I intend to mount two antennas of the same band near each other - though, unlike in a repeater application or in your own, I do not intend to actively use both antennas at the same time. Nevertheless, there may still be some passive interference from an 'inactive' antenna' mounted near an 'active' antenna.

One antenna will serve a (1) TX/RX transceiver and another will be connected to (2) a RX-only device (that will likely be unusable while the TX/RX antenna is radiating - and I can accept this).

But my question about the effectiveness of a transmitting antenna in proximity to another resonant antenna remains: How much separation is needed between two identically-tuned antennas to avoid coupling between the TX'ing antenna and the nearby, second resonant antenna that will inevitably be passively receiving, whether I like it or not?

I understand that the second resonant antenna may couple and 'suck' the radiation from the TX'ing antenna away from the rest of the outside world, compromising the effectiveness of my transmissions.

Lauri - What guided you to 26" of separation? Was there some rule that could be applied in the context of two same-band antennas?

 

[Lauri-CoyoteFrostbite]

Ok, I think I have the complete picture

First off, a caveat--- during my working years I had charge of a small fleet of vehicles we used at a National Laboratory-- with radios/RF sources installed galore- so this is/was a subject I was painfully aware of.

Since you don't plan on using both antennas at the same time, I see no reason to be concerned about de-sense issues. Of course it goes without saying you want to separate them as much as possible---- in my example of my Wrangler I chose a compromise. I wanted to separate them that "much as possible, " yet still retain the element of esthetics.
26 inches, or so, was a fine distance-- and low profile Motorola quarter waves was a natural choice.
Sorry, but I have no formulae for calculating that best separation.

I have used gain antennas on many vehicles in the past, but to be honest, I have found them not any great an improvement over the humble quarter wave (I can sense a dissonance gathering in the gallery .)
Quarter wave's are easy to install, maintenance free...etc. etc. As far as 'gain' is concerned, my philosophy today is if I can't talk to someone with 20-30 watts and a quarter wave antenna then I won't be talking to them. This limitation has seldom been a problem- but I live in mountains where signals are either there or they're not- angles of radiation are a mixed blessing or a curse... you just live with it.

Yes, there maybe an interaction between two similar antennas, but one will not suck out power from the other. There may be a radiation pattern shift slightly but this is a mobile installation- if a minor lobe disruption causes issues, there is more at work than the antenna patterns. Move the vehicle, increase the power---

I was fortunate that for years we at my lab had access to a antenna testing range. We were able to actually see these various parameters live; and thus run various experiments.
I would suggest that if you are really curious about theses antennas that you construct some sort of a test arrangement to see what the effects can be. A sheet of metal as a ground plane, antennas mounted on it in various configurations-- power on and see what the results are. It can be as fancy or as simple a setup as you desire, but easier and more insightful than drilling holes in your Jeep- a Beater or no--- first.

Good Luck, Cowboy !


Lauri






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[Tobydog]

There is a chart on Google, type in antenna separation by frequency and it should give you some guidance for distances.