dual antenne q?

[squirrellsgnwild]

How far away do the two antennas have to be to be worth it to use two?

[L.B.]

Originally Posted by squirrellsgnwild

How far away do the two antennas have to be to be worth it to use two?

I think the optimal distance between dual antennas is supposed to be 8 or 10 feet.

[kd5drx]

[LINK POSTED BY MEMBER] Only Members Can View This Truck Forum Link.
Try this post this has been beat to death

[BobC]

I wish I still had the extra land to create an antenna test range for vehicular antennas.

I think I coulda made a little extra money with something like that.

Would have been interesting to see how much of an improvement different experiments could make. Or at least dispell some myths about single & dual antennas.

[kd5drx]

it doesn't take that much land we did on a 5 acre parking lot with a good FS meter you can see the difference in about a 200 ft radius with a good FS meter calibrated and a couple of different set ups on a test stand or platforms we did it years ago and proved the theories as outlined on the computer models it is real easy to use the computer models and then duplicate them in the field. It showed exactly what the EZNEC program had shown to be the radiation pattern once we added some attenuation to the FS meter to make up for the closeness. A dual antenna system set up in co-phase has a directional effect front to back almost like a 3 element beam with out the secondary lobes. And has a little larger side lobe effect. Now a single antenna placed in the middle of a large vehicle with lots of flat surface has the best over all radiation pattern but on a Semi it presents a big problem with hight since most of them are already 13"6' tall. So the next best is a single antenna on one side or the other then you have allot of shadows and minor lobes and dead spots on the opposite side of the antenna. So over all if you want to talk up and down the road your on then the co-phased has an advantage but if you want to talk to the opposite lane ahead of you the the drivers side is the best placement of a single antenna as it radiates front and left side with the right back side almost completely blocked up close. now this goes away the further you get away from the vehicle like 10 or 20 miles or further. Thats been my experience after spending about 6 months playing with different set ups on different vehicles that i owned in an open 5 acre lot and with certified test equipment and O scopes and signal generators and FS. meters. We never used a radio only a signal generator that we could control exact amounts of power and a known signal quality that would not change.

[BobC]

Yes, I'm aware of the patterns that should be there. I"ve used EZNEC before or at least a derivative. I've found that the programs' ok for general antenna use but lacks in modeling for vehicles with irregular ground planes. It may be that there's a model for it now but I haven't bothered to check if there is.

I was thinking more along the lines of several equidistant telephone poles with a couple or three NIST standard antennas & detectors fixed on each. Or I was considering 1 NIST antenna per pole that could be motorized up & down the pole to gauge the various angles of radiation in the more important planes.

They'd be just a few wavelengths apart & hopefully outside the bulk of the near field area.

I dunno if I can get them 10 wavelengths out & still have enough height on the poles to catch the higher radiation angles. I haven't done the geometry yet nor have I decided yet what is the highest radiation angle that I'm interested in knowing about.

The detectors & antenna control would be linked back by fiber on appropriate mux/demux & I'd probably use NI's Lookout software to build a GUI & metering/control/sync functions.

I'd probably want to create a few other NIST antennas to perform similar tests on other bands.

You see, I have grander visions than some but I figure if I'm gonna' do it, I'm gonna do it as best I can.

[markthetrucker]

Originally Posted by kd5drx

it doesn't take that much land we did on a 5 acre parking lot with a good FS meter you can see the difference in about a 200 ft radius with a good FS meter calibrated and a couple of different set ups on a test stand or platforms we did it years ago and proved the theories as outlined on the computer models it is real easy to use the computer models and then duplicate them in the field. It showed exactly what the EZNEC program had shown to be the radiation pattern once we added some attenuation to the FS meter to make up for the closeness. A dual antenna system set up in co-phase has a directional effect front to back almost like a 3 element beam with out the secondary lobes. And has a little larger side lobe effect. Now a single antenna placed in the middle of a large vehicle with lots of flat surface has the best over all radiation pattern but on a Semi it presents a big problem with hight since most of them are already 13"6' tall. So the next best is a single antenna on one side or the other then you have allot of shadows and minor lobes and dead spots on the opposite side of the antenna. So over all if you want to talk up and down the road your on then the co-phased has an advantage but if you want to talk to the opposite lane ahead of you the the drivers side is the best placement of a single antenna as it radiates front and left side with the right back side almost completely blocked up close. now this goes away the further you get away from the vehicle like 10 or 20 miles or further. Thats been my experience after spending about 6 months playing with different set ups on different vehicles that i owned in an open 5 acre lot and with certified test equipment and O scopes and signal generators and FS. meters. We never used a radio only a signal generator that we could control exact amounts of power and a known signal quality that would not change.

I agree almost completely, but If you want to get out further and are using a little power on a big rig. like an RCI 2970, wouldn't the passenger side work better by using the ground plane of the vehicle? (flatbed as well so usually not 13ft)
I don't know I have seen other computer models that show this effect on the signal. I have always been told that this was the best way to run a single. I run co phase so just curious about your thoughts.

[BobC]

Most commonly, your antenna will radiate in the direction it has the best ground plane metal attached to it.

If you have an antenna mounted on the rear of a station wagon, the best pattern will be to the front of the vehicle.

It works similarly on a truck with some exceptions.
Most trucks don't have a metal body or roof line.
You need to make or include as much ground plane as possible at the base of the antenna.

KD5's conclusions seem counterintuitive but I can see what's happening in his tests.

When you have an antenna mounted on the passenger side, the box can get in your way & shield your antenna from units on the opposite side of the road.

If you have a poor groundplane at the passenger side antenna mount, it won't do well in the driver's or forward directions either.

By his placing the antenna on the drivers side, he "opens" the area to the opposite side of the road.

No more obstructions from the box & the antenna can radiate using the rather non-directional aspects of a mirror bracket for a groundplane.

If the truck had a metal roof line which was electrically tied into the system, the passenger side antenna could produce a better signal in the drivers & forward directions than one on the drivers side.

It could still have a problem to the opposite rear because of a box though.

Trucks are a bit of a pia when you're trying to get maximum snot out of the antenna system.

[ee_john]

Originally Posted by squirrellsgnwild

How far away do the two antennas have to be to be worth it to use two?

This is an excellent question. My site has some simulations of two antennas fed in phase. Generally if the two antennas are within about 5 or 6 feet of each other at CB frequencies the two act almost like one with no real benefit except that two antennas might help getting more signal around the tall vehicle body.

As the separation continues the pattern loses some gain in-line and raises the gain broadside.

At about 13 feet this pattern has the most elongated oval pattern and then starts to develop deep nulls as the separation continues.

The eight or nine feet separation found on trucks makes some sense to concentrate some additional energy up and down the highway.

Another way to put this is two 27MHz antennas placed on each side of a vehicle legal to drive on US highways will work pretty well.

I think they look pretty cool too.

Don't forget to ensure both antennas are fed perfectly in phase with a co-phase harness.

I will be modeling a typical truck body to further refine these simulations, but have to wonder how much of a truck's skin is still made of metal.

John