Distance between antennas

[Blunt630]

What would be the best distance between two antennas both scanning 700mhz scanners p2 systems. Using these antennas. TRAM 1091-BNC Scanner Trunk/Hole... https://www.amazon.com/dp/B01DY8FTR8?ref=ppx_pop_mob_ap_share

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[KB4MSZ]

A full wavelength or greater would be best, about 17"

 

[mmckenna]

For receive only, it's not super critical, unlike if you were using transceivers.

What you want to be careful of is making one of the antennas act like a reflector for the other one, essentially making it directional.

at minimum 1/2 wavelength or more should be OK. At 700MHz that would be around 7 or 8 inches. Full wavelength would be "more better". If you are not pressed for space on your vehicle, spread them out.

 

[phyberoptics]

If both antennas are simply receiving, there is no minimum distance required.

 

[Blunt630]

So far they are close. I will finish plasticdipping the last bracket this weekend.

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[mmckenna]

Any particular reason you want them that close together? If it's as close as it looks in the photo, you are probably making them slightly directional. If the signal is good and strong, unlikely you'd notice any impact. If you are on the fringes of coverage, some more spacing may help.

 

[KB4MSZ]

Looks like they may be in need of a bit better overall grounding as well.

 

[mmckenna]

Two other items to be aware of….
Those style NMO mounts are designed to be mounted with the coaxial cable inside the vehicle where the connection is dry and protected from the elements. Using them exposed like that can create issues with moisture, road salt, oil, etc. That can lead to corrosion, water intrusion into the cable, etc.
Seal up the underside of the NMO mount very well and it'll last a lot longer.

As for the Tram/Browning product, be careful of the connectors on the end. They are not great connectors and usually not crimped well. The strain relief is pretty much what holds them together. Had someone bring me one of those to install on a truck. I had to cut the connector off to route the cable. Reinstalled a new connector and it was fine. Later, I disassembled the UHF connector out of curiosity. Poorly crimped, cheap connector. If you start to have issues down the road, consider the connector has failed.

 

[Blunt630]

Just set them as a starter distance. I have plenty of whip to go 7 or 8 inches.

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[Blunt630]

Ive had the Tram though window antennas that are still working great never fallen off for the 6 years it been on. Even through the weather we have here in Chicago

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[duffy109]

Hey, why not screw the angle mounts right into the square cross beam instead of the rust able brackets and non-stainless steel screws?


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[prcguy]

I'm with the person that recommended a full wavelength or greater. At 1/4 wavelength apart or about 3" the pattern of each antenna will be skewed in one direction. At a half wavelength or about 6" each will have a slight clover leaf pattern. At one wavelength each will have a pattern with eight lobes. With each slight gain lobe there will be an equal amount of lossy nulls. So get them as far away from each other as possible to reduce the interaction.

Edit: I would also add a thin 6" dia aluminum disc under each antenna mount to provide a ground plane, as they are way too far above the roof to have a valid ground plane. You can usually get pre cut 6" aluminum discs on Ebay for reasonable prices.

 

[Ubbe]

Take a metal wire, a straighten out coat hanger or something about the same lenght as your antennas. Tune to an ANALOG signal that are pretty weak with some noise. Hold that metal wire about the same distance from the antenna as the other antenna are and move the wire around the antenna and listen to the signal. You'll probably can't hear any difference and comming closer to a cell tower will have much more impact on the signal. Install as you find most conveniant. I've seen several boat antennas installed that way on a piece of small metal and a skipper had no complaints with his VHF radio for at least 10 years with the car antenna mount being exposes to salt water winds.

/Ubbe