multiple antenna on a mobile

[BOOTY MONSTER]

ok , im starting to get a intrest in running more than one antenna on my montero . im not interested in the key down thing with 4 or 5 whips . just wondering how much 2 antennas work better than one . i understand it makes the signal directional like a beam . ive herd the term 2 hott and also about one antenna only being grounded . how about a lil basic edumacitation about this in laymen/cb'er terms ?

 

[W5LZ]

Booty M',
Using two antennas does make for a sort of directional radiation pattern similar to, but not really as directional as a beam. That pattern can be roughly fore/aft like the direction the vehicle is traveling, or side/side. Just depends on how the two antennas are fed in relation to each other. That 'directional' pattern can also be 'steered', or switched between the two. The 'catch' with that directional pattern is that it'll be in the directions you want only half the time, or in the case of the 'steerable' pattern it'll mean you'll be flipping that switch quite often every time you change directions when driving. The 'other' thingy about dual antennas is that matching everything isn't going to be very simple/easy, and the SWR will never be below about 1.5:1 (if that low). All of that assumes that you are feeding both antennas, 2 'hots'.
If one antenna is 'hot' and the other grounded, it'll act similar to a two element beam, sort of. That arrangement can also be switched, but it'll mean three antennas and twice the feed lines. 'Do'-able? Sure.
Notice the "similar", and not "like", a beam antenna. that's cuz of the arrangement's height above ground (among other things).
Another thingy is the separation between antennas. The so-called 'perfect' distance between dual antennas is about 9 feet. More or less than that and things are compromised to some extent, not quite as 'good' performance as expected. The directional pattern would resemble a football more than a figure-8, sort of.
All of this is a generalization, never always exactly 'true' depending on gobs of things. But, in general, still the way things are. In most cases, dual antennas are not worth the effort except for very specialized instances/circumstances. Lots of myths about dual antennas, bet you've heard most of them. Wanna try them? Great! Have at it. I think you'll come to the same conclusions that I have.
- 'Doc

Oh yeah! They also mean twice the expense...

 

[Pushrod]

I asked this same question on another board recently. A member there copied and pasted this from somewhere on the web. He did not say where it came from. Does this information sound correct?

"First step is to determine if the second antenna is going to be a passive element, much like that of a base station beam, or will both antennas be active? For an active system where coax will be strung to both antennas and the RF fed to both, The minimum spacing will be 1/8 wavelength. That will be about 54.5 inches at 27.105 mc. That is the easy part. The hard part is to decide if you want the main antenna fed in phase or out of phase. If out of phase, then by how much?



For Antennas spaced at 1/8 wavelength apart: ~ 54 inches in phase 0.3 dB gain omni pattern,

45 degrees 0.9 dB gain somewhat less omni

90 degrees 2.3 dB gain more pronounced lobe in the line of the antennas

135 degrees 4.2 dB gain diamond pattern, 180 degrees 3.8 dB gain bidirectional, figure 8 pattern



For antennas spaced 1/4 wavelength: ~ 108 inches,

In phase 1.1 dB gain somewhat flatten pattern perpendicular to the array.

45 degrees 1.6 dB gain still perpendicular to the array with some energy off the front.

90 degrees 3.1 dB gains again a diamond pattern in line with the array.

135 degrees 4.5 dB gain again the pattern generated looks more like a mushroom

180 degrees 3.6 dB gain bidirectional in the line of the two antennas.



There are patterns for 3/8 wavelength spacing. If you have a vehicle bigger than a compact car then those will yield similar patterns and gain. Now how do you accomplish the phasing? The easiest is to use lengths of coax that are carefully measured and cut to the desired lengths.

For 0 degrees both coaxes are the same length

45 degrees one coax will be 1/8 wavelength longer.

90 degrees one coax will be 1/4 wavelength longer.

135 degrees one coax will be 3/8 wavelength longer.

180 degrees one coax will be 1/2 wavelength longer.



Both coaxes will be 50 Ohms. It is best to make the coaxes 1/2 wavelength. The longer coax or the one that will phase shift will be longer by the lengths above.



The above method may not yield the gains or patterns listed due to phase errors in the antenna currents. This is due to the phase shifts of the voltage and the current in the coax which will vary with length and load. This method while the easiest generally yield the widest variation from the listed patterns and gains.

The best method is feed both antennas with 1/4 wavelength coax. At the junction where the main feed tees in, place a series inductor and a shunt capacitor in one of the feed lines to one of the antennas. By making the inductor or the capacitor variable or both, the phase of the current in one antenna can be adjusted to a pattern and gain that you wish. This method requires more construction techniques and some means of monitoring the phase adjustments.



It is possible to get as much as 4.5 dB gain by phasing two or more antennas on a vehicle. With the first method you get what you get. It is dependant on how well you measure the coax and the antennas themselves. Also a word of caution. Use the same type of coax for both legs of the phasing harness to each antenna. You must also account for the velocity factor of the coax. This also adds a variable that is not adjustable in the first method."

 

[BOOTY MONSTER]

thanks guys .

 

[psycho]

ok , im starting to get a intrest in running more than one antenna on my montero . im not interested in the key down thing with 4 or 5 whips . just wondering how much 2 antennas work better than one . i understand it makes the signal directional like a beam . ive herd the term 2 hott and also about one antenna only being grounded . how about a lil basic edumacitation about this in laymen/cb'er terms ?

If you run a hot antenna up front and a grounded reflector in the rear........ the front would obviously be the strongest but the sides are stronger too with the reflector. The only side really cut off is to the rear of the vehicle. IMO 2 or more antennas on a vehicle is ridiculous looking but it is for a purpose. I gain 2 S-units with my back door set up, but I only use it maybe 2 times a year.

 

[74IN]

I had a Nissan Pathfinder with a Wilson 1000 magmount on it.

I added a 102" whip with two 6" springs on it, mounted to the spare tire carrier.

Ungrounded, nothing.

Grounded it, it became more directional than I expected.

Worked great, directional towards the front.

I was going to add a switch to the ground wire, to switch back to omni, sold the thing before I got around to it.

 

[mackmobile43]

I had a Nissan Pathfinder with a Wilson 1000 magmount on it.

I added a 102" whip with two 6" springs on it, mounted to the spare tire carrier.

Ungrounded, nothing.

Grounded it, it became more directional than I expected.

Worked great, directional towards the front.

I was going to add a switch to the ground wire, to switch back to omni, sold the thing before I got around to it.

I got the same results by mounting a predator 10-k to the far rear of my pickup on the bed rail and the hole was already there from the previous owner.

 

[ROC1]

I had a Nissan Pathfinder with a Wilson 1000 magmount on it.

I added a 102" whip with two 6" springs on it, mounted to the spare tire carrier.

Ungrounded, nothing.

Grounded it, it became more directional than I expected.

Worked great, directional towards the front.

I was going to add a switch to the ground wire, to switch back to omni, sold the thing before I got around to it.

74 when you say "nothing"... you weren't running any coax to it? It was just sitting there on the spring with nothing but a ground wire...

I'd like a setup that I could throw on the S10 for more directionality and some additional gain when I'm out shooting skip...

How would differences in height affect the active and passive antenna's... I run a 102" whip on top of the cab, the antenna off the rear bumper would not ultimately reach the same height...

 

[ROC1]

okay...

I'm actually going to play around with this a little... I started with an in phase setup as it would eliminate a few variables at the start. Once I get this setup running right I will try the out of phase setup.

I'm doing this in less than a perfect setup and I'm sure it will bugger me up...

Right now I've got the Monkey Made on the Roof Magmount and the 102" whip attached to the taile gate, in line with the other antenna...

The whip sits quite a bit lower than the MM, and I'm sure that will screw with things...

The MM is getting a much better SWR than the whip... 1.1 vs 1.5...

With them both recieving signal the swr jumps to just below 2.0

The whip location needs to be grounded better and I will tackle that this weekend and see if I can't get that spot more in line with front.

If nothing else its pretty interesting stuff...

 

[W5LZ]

Pushrod,
It sounds creditable until the part about it all being 50 ohm coax. At that point, it isn't creditable anymore.
- 'Doc

 

[ROC1]

Pushrod,
It sounds creditable until the part about it all being 50 ohm coax. At that point, it isn't creditable anymore.
- 'Doc

For the ignorant can you explain how that throws it off... thnx

 

[ROC1]

When running the out of phase setup should we be running the longer coax to the front antenna or to the rear, or does it not matter?

The details given mention the patterns "figure eight" etc, but don't seem to indicate if there is a front to rear bias in the pattern created by the location of the phase shifter...

It would be easier for me to run the 1/2 wave difference to the rear of the rig, althought it's not a huge issue.

I did get the two antenna running together to an SWR of just above 1.5, prob 1.6 or so... I'm still concerned about running power at those levels so I'll work on it just a little more today; I need to ground the tail gate to the bed and the bed to the frame and then see what we have...

 

[W5LZ]

ROC1,
If you place two 50 ohm resistors in parallel, the resulting resistance is 25 ohms, right? Same basic thing with impedances. If two 50 ohm impedance antennas are connected in parallel (at a 'T'), you do not end up with 50 ohms of impedance, it's closer to 25 ohms. That means that the SWR at that point is something like 2:1, not exactly 'good'. That's why most phasing harnesses are made from 75 ohm coax. Still won't end up with 50 ohms, but it'll be much closer. See where I'm going?
- 'Doc

(If you can find it, 93 ohm coax makes for very nice phasing harnesses. Not the 'best', but closer to it.)

 

[ROC1]

ROC1,
If you place two 50 ohm resistors in parallel, the resulting resistance is 25 ohms, right? Same basic thing with impedances. If two 50 ohm impedance antennas are connected in parallel (at a 'T'), you do not end up with 50 ohms of impedance, it's closer to 25 ohms. That means that the SWR at that point is something like 2:1, not exactly 'good'. That's why most phasing harnesses are made from 75 ohm coax. Still won't end up with 50 ohms, but it'll be much closer. See where I'm going?
- 'Doc

(If you can find it, 93 ohm coax makes for very nice phasing harnesses. Not the 'best', but closer to it.)

Absolutely... thank you.

Both of my antenna in this little test were well below 1.5 but together were well above... i've read that would happen but didn't know exactly why. (and I had to find test for myself). If I really want this to work I will need to re-cable... I will go on a hunt for the 93. I just don't see my local guys having it... will check with HRO.

Thanks again...

 

[711]

I got an 2000 Explorer, I set the two antennas 6ft apart .The back antenna is 4 inchs shorter then the front antenna. The coax is the same length 9913.I do have 12 ft to the amp. My SWR is 1.4 back swing to 1.2. I still get RX and TX from the back and the side but I do not know if my directivity(is that a word) is more in one direction or if I'm still omni direction and pulling close to a figure 8 pattern. I read that 10 ft of 66% VF coax is about 150 degree phase shift between the 2 antennas and you get the 4.5db gain.in phase (no shift 1.1db gain) So I am still expermenting too.