Mobile Co-Phasing

[fmulder13]

Hey all,

I'm revamping my mobile setup, and am looking for some co-phasing advice. Since I'm interested in monitoring almost only the VHF band, I'm going to run a Motorola Spectrum 5/8 on a mag mount to my Pro-2096. I'm considering buying a second Spectrum, and running it on another mag mount to my scanner with a RadioShack BNC T-adapter. As far as I've been able to determine about co phasing, I need identical antennas (check), identical lengths of coax (check,) and the placement must be correct (check. I want to place them both on the trunk, and will probably put them right on the edges on opposite sides of the trunk.)

Assuming I'm correct on my co-phasing technique, my question is thus. I've read that co-phasing two antennas will create a more directional pattern that is parallel to the car. I've also read that on a mobile setup, since the ground plane of a vehicle (the metal body) is so uneven co-phasing will actually create a more omnidirectional pattern. Any thoughts on whether this is true?

Most of the signals I'm trying to receive are fairly strong, but I want to see if I can't pick up my local PD when they're not using the repeater on their portables. Would co-phasing help with this or am I just out of luck and shouldn't waste my money on a second Spectrum?

 

[iMONITOR]

I think you'd be waisting your money. Co-phasing was (maybe still is) popular with a lot of CB'ers, but if it was that effective I think you'd see a lot of hams doing it, and I don't.

 

[N_Jay]

Co phasing (if done right) gives you just a tad of gain (3 dB) in two directions, and gives you big nulls (10 to 20 dB) in the other two directions.

Done wrong you get nulls with even less gain.

Is is for truckers so they can get a few dB to the front and back, and to get rid of "little jimmy on his new toy" to the sides.

 

[policeMonitor]

well if ya know anything about radios 1 antenna is better then 2 unless you want to be directinal most drivers run 2 antennas for looks but only 1 is hot but alot of guys only run 1

 

[Don_Burke]

I would look into the possibility of an antenna optimized for those portables and switching back and forth between the two as needed.

Phased antennas have advantages. Forward gain is not one of the big ones.

 

[fmulder13]

I would look into the possibility of an antenna optimized for those portables and switching back and forth between the two as needed.

Phased antennas have advantages. Forward gain is not one of the big ones.

What sort of antenna would that be? Any ideas? I personally observed an officer in his car able to talk to his sergeant who was about a half-mile away on his portable using the talkaround while the officer was about two feet from my car. I couldn't hear the sergeant but it was clear from the officer's traffic that he could hear him loud and clear. The department uses 1/4 waves mounted on their trunk

 

[Don_Burke]

What sort of antenna would that be? Any ideas?

Mobile, I would find an antenna I could cut to be an unloaded 5/8 wave on the frequency of interest. From a fixed location, I would make a J-pole.

I personally observed an officer in his car able to talk to his sergeant who was about a half-mile away on his portable using the talkaround while the officer was about two feet from my car. I couldn't hear the sergeant but it was clear from the officer's traffic that he could hear him loud and clear. The department uses 1/4 waves mounted on their trunk

They also use receivers considerably better than scanners.

 

[k9rzz]

Personally, I think it would be a fun experiment. Just because no one is doing it, doesn't mean it wouldn't be worth while to play with. The only thing that bugs me is that I would think that mounting the antennas on the trunk, the car itself would distort the pattern to the front. If they are mag mounts, then you have total freedom in chosing placement and spacing and that sounds like fun!

John K9RZZ

 

[N_Jay]

Personally, I think it would be a fun experiment. Just because no one is doing it, doesn't mean it wouldn't be worth while to play with. The only thing that bugs me is that I would think that mounting the antennas on the trunk, the car itself would distort the pattern to the front. If they are mag mounts, then you have total freedom in chosing placement and spacing and that sounds like fun!

John K9RZZ

1) if no one had experimented with it, then it might be "fun", but you are simply "experimenting" in an area that has been well studied, and is well understood.

2) unless you put significant effort into your measurements you will have no way to know the results of your "experiments".

This is akin to doing a different dance each day to see which one produces the most rain.

 

[fmulder13]

Mobile, I would find an antenna I could cut to be an unloaded 5/8 wave on the frequency of interest.

From what I understand don't all 5/8 waves rely on their loading coil to work? Since 5/8 isn't a multiple of 1/4, the whip is cut to be 5/8 of the wavelength while the coil electrically adds length so the antenna actually functions as a 3/4 wave.

 

[Don_Burke]

From what I understand don't all 5/8 waves rely on their loading coil to work? Since 5/8 isn't a multiple of 1/4, the whip is cut to be 5/8 of the wavelength while the coil electrically adds length so the antenna actually functions as a 3/4 wave.

No, a 5/8 wave all by itself is a resonant antenna. There is a matching circuit involved, although some just tolerate the mismatch.

http://www.qsl.net/w4sat/five8th.htm

http://users.belgacom.net/hamradio/schemas/on6muvhf58verticalantenna.htm

http://www.scanshack.com/antcal.htm

http://www.sbe47.org/download/teslaant.txt

 

[Halfpint]

Co phasing (if done right) gives you just a tad of gain (3 dB) in two directions, and gives you big nulls (10 to 20 dB) in the other two directions.

Done wrong you get nulls with even less gain.

Is is for truckers so they can get a few dB to the front and back, and to get rid of "little jimmy on his new toy" to the sides.

{YIKES!} Again! I find myself agreeing with N_Jay! What's this world coming to!? Hmmm.... Maybe they've started putting something new into the water? It sure, AFAIK, *hasn't* been anything I've smoked recently. {CHORTLE!}

Just an `Olde Fart's' 2¢ worth. {VB GRIN!}

 

[N_Jay]

{YIKES!} Again! I find myself agreeing with N_Jay! What's this world coming to!? Hmmm.... Maybe they've started putting something new into the water? It sure, AFAIK, *hasn't* been anything I've smoked recently. {CHORTLE!}

Just an `Olde Fart's' 2¢ worth. {VB GRIN!}

No problem.

You will find the more you learn and the more logic you apply, the more often you will be agreeing with me.:twisted:
:lol: :lol: :lol:

 

[fmulder13]

The only reservation I have left about my idea is that from time to time I see two identical antennas mounted on state police-type cruisers that, based on their placement I'd have to believe that they're co-phased. Anybody else seen this, or can someone explain away my doubts? I have seen the 4 antennas mounted on the roof, and I imagine there's some reason for co-phasing there (would 4 make the radiation pattern more omnidirectional than two?) but I've also seen two completely identical VHF antennas mounted in a way that would make me believe that they're being co-phased.

 

[kb2vxa]

Hi 13 and all,

Oh that term "co-phasing", what a misnomer! There just ain't no such animal, no co about it! It's a phased array and that's about it, useful ONLY in fixed locations, and it's essentially a "beam" antenna. Phased elements driven or parasitic as in a Yagi serve to aim the signal in one or more directions depending on the physical arrangement of the elements and the phase relationships between them.

The CB "Twin Truckers" approach is the worst hoax ever perpetrated and is sheer fallacy, no way can an end fire array counteract the asymmetrical ground plane effect of a vehicle body. No way can proper spacing be achieved on 11M and in any case although it can on VHF still it's pretty stupid considering the fact it's MOBILE. Turn the vehicle and you rotate the pattern, DUH!

The closest mobile installation I have seen to an electronically steerable array was produced by Antenna Specialists which had at it's heart a rotary switch that selected different lengths of cable in order to change the relative phasing of two quarter wave antennas spaced a quarter wave apart. Naturally 9' spacing could only be achieved by mounting one antenna on the rear quarter panel and the other on the opposite fender of a 70s "land cruiser". Still only the omni position was effective while the aircraft carrier on wheels was in motion.

OK, now it's time for you to crack the books and learn some basic antenna theory. Sorry, even with a field as narrow as phased arrays there's WAY too much to write here and I have serious doubts you have a Greek character set installed, you can't handle the math. (;->)

"most drivers run 2 antennas for looks but only 1 is hot"

Now there's a case for Dumb and Dumber. Having a parasitic element in the near field distorts the pattern just as much as a driven element. Uda and Yagi based their antenna on this particular law of physics.

"Personally, I think it would be a fun experiment."

Personally I think reinventing the square wheel is a gross waste of time.

"From what I understand don't all 5/8 waves rely on their loading coil to work?"

What you don't understand is it's an impedance matching transformer, not a loading coil, the radiator is already 5/8 wave.

"Since 5/8 isn't a multiple of 1/4, the whip is cut to be 5/8 of the wavelength while the coil electrically adds length so the antenna actually functions as a 3/4 wave."

Totally wrong, do the math. Since when does 5/8 equal 3/4? A 3/4 wave radiator is too long, it gives a funky vertical takeoff angle so it's not normally used. The exception is the ham who wants a 1/4 wave 2M antenna to function as a dual band antenna, it's 3/4 wave on 70cM giving a good 50 ohm match on both bands.

"There is a matching circuit involved, although some just tolerate the mismatch."

EH?! That "some" surely doesn't include the finals that just went south for the winter and never came back.

"I find myself agreeing with N_Jay! What's this world coming to!?"

It's come down to me in the first sentence, the rest falls flat. How can nulls exhibit gain, they're NULLS! (Null value = 0) Then if truckers want more signal front and rear than side to side the'd rely on the ground plane effect, the "theory" behind two antennas is to get more to the sides.

"Hmmm.... Maybe they've started putting something new into the water?"

LSD

Like I said, crack the books Yogi, you're not smarter than the average CBer YET!
(Awaiting the usual torrent from N_Jay, hi.)

Edit;
13, the two identical antennas are for two radios operating on the same band but on different radio systems. The "4 square" arrangement is for the LoJack receiver, there you have a truly directional array and then some. Without getting too technical, the system senses the phase differences between the antennas using the Doppler principle thus locating the direction the signal is coming from.

 

[Don_Burke]

EH?! That "some" surely doesn't include the finals that just went south for the winter and never came back.

This is a receive only installation. The only "finals" are in the audio amp and there is little an antenna can do to them. You might wish to read this thread again to get up to speed.

It's come down to me in the first sentence, the rest falls flat. How can nulls exhibit gain, they're NULLS! Then if truckers want more signal front and rear than side to side the'd rely on the ground plane effect, the "theory" behind two antennas is to get more to the sides.

Therein lies the problem. You are starting with a flawed premise, which guarantees you will come to a flawed conclusion.

Open road CB is not like two meters and treating it the same can take you down the wrong path.

The primary problem (at least on the receive end) of CB on the road is rejection of unwanted signals. On the road, the vast majority of signals the operator cares about are coming from pretty much directly in front of him or directly behind him since those are the stations he can exchange road condition information with. The signals from the side are more often than not useless base station chatter.

The twin antennas are not intended to achieve forward gain, although they are often marketed that way. Side rejection is the primary benefit of the "twin truckers" and is noticeable at 27MHz at a separation of about seven feet, which can be exceeded with mirror mounts. Granted, a full null can not be achieved at less than about eighteen feet. I have not plotted out what the gain would be for a separation of seven feet and expect it would not be worth the trouble since best case gain is only a couple of dB. (As I recall, the actual gain peak is at a separation of 210 degrees. Don't quote me on it as it has been a while.)

If the goal is to chat with anyone, GMRS or amateur radio would be a better choice.

In any case, phased verticals have little application in mobile use aside from things like Lo-Jac receivers.

 

[k9rzz]

This is akin to doing a different dance each day to see which one produces the most rain.

I couldn't disagree more. Why turn on the radio? You know what you're going to hear, right?

John K9RZZ

 

[N_Jay]

. . . . .
Like I said, crack the books Yogi, you're not smarter than the average CBer YET!
(Awaiting the usual torrent from N_Jay, hi.)
. . . . .

Not worth the work with your refusal (or inability) to use teh quote feature.

 

[N_Jay]

I couldn't disagree more. Why turn on the radio? You know what you're going to hear, right?

John K9RZZ

So, just exactly how are you going to test each of your configurations?:roll: :roll:

(Experiments without valid tests are back to rain dances)

 

[kb2vxa]

Hi Don and all,

OK, I was wrong about it being receive but still a mismatch of such magnitude results in considerable loss. That's why we use antenna tuners and the more effective preselector.

"Therein lies the problem. You are starting with a flawed premise, which guarantees you will come to a flawed conclusion."

It ain't necessarily so, null value equals zero in any math book.

"Open road CB is not like two meters and treating it the same can take you down the wrong path."

Who said I was treating it like 2M? Being I was licensed as KMD7606 in 1965 and KB2VXA in 1995 please don't put the cart before the horse. I have years of knowledge and experience beginning in 1958 and have been CB on the open road using more antenna systems than most. Naturally they're worlds apart, HF is a long way from VHF in many respects. For that matter directional arrays on CB came from Amateur Radio long before the FCC gave our 11M band over to Class D CB to replace the old UHF Class A CB that was reallocated to public service. It was the biggest mistake in radio history from even a CBer's point of view, one who doesn't wish to fight it out with "skipland" for one but I digress. Maybe one of these days we can discuss a 40M vertical phase steerable array but it would only bore the scanner buffs. Well, maybe the Super Scanner CB antenna that worked along the same lines. (;->)

"If the goal is to chat with anyone, GMRS or amateur radio would be a better choice."

GMRS is inadequate for the long haul, especially since many repeater owners restrict access but Amateur is excellent for regional VHF/UHF and you can get some pretty good mileage, DX even on HF. With the linked repeater systems out in those wide open spaces of the Southwest you can get coverage rivaling 40M. There are plenty of interstate truckers on 20M for example, I've even heard an aeronautical mobile hauling freight in an old Douglass DC-3, and I thought the dinosaurs were extinct. Speaking of /AM, once I heard a commercial airline pilot working 20 under his ham callsign, the HF radio easily covers the ham bands.

It makes fighting it out on channel 19 look like the waste of time it is although it has certain advantages, like more truckers. That's why I can't put down CB entirely, my friend Jim W2RXR being a trucker has his HF/VHF/UHF ham rig and a CB in the cab.

"In any case, phased verticals have little application in mobile use aside from things like Lo-Jac receivers."

How very true! FYI, LoJack doesn't use a phased array, it uses the Doppler principle. Hams have copied it and now we have one of the most effective RDF systems available. It's SO much better than the old loop, or even sticking a Yagi out the window, never mind the clumsy old goniometer DDRS. Now if you want to go back to the future theres always HFDF or huff duff as we used to call it. (;->)

Oh 13, are you >thoroughly< confused now? (;->)