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Author Topic: 18 Wheeler Co-Phased Whips  (Read 2243 times)
KB5TJ
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« on: November 25, 2007, 04:31:07 PM »

This is partly a learning project prior to tackling a Yaesu FT-857D installation in an 18 wheeler.  I have replaced the stock mirror-mount CB antennas with a co-phased 4 ft helical wound whip setup mounted directly to the each side of the cab, bases approx 9 ft above the ground (I know this is not the ideal setup but that's all I have to work with).  The coax ground is affixed to the aluminum sides of the cab at the antenna feedpoints (the sides, back, and bottom of the cab are aluminum, comprising perhaps 130 sq ft of surface area, likely minimally grounded to the truck frame -- this is all the ground plane I have to work with).

The retail packaged co-phase harness is 18' of RG-59A/U 72 ohm foam on each leg (which, adjusted for a generic velocity factor of 0.66 -- which may not be right for foam -- should be about 3/4 wavelength). I installed everything as supplied and get the following readings on an MFJ-259B connected to the 50 ohm input to the co-phasing harness (PL-259):

FREQ . / R . / X ./ SWR / Imp./ Phase

26.965 / 029 / 29 / 2.7 / 041 / 44 deg.

27.185 / 067 / 48 / 2.4 / 080 / 36 deg.

27.405 / 144 / 27 / 2.9 / 146 / 12 deg.

This is my first use of the MFJ-259B, and I am at a bit of a loss (no pun) as to the readings.  The 2.4 SWR at the middle freq above seems reasonable for this far from ideal setup, and it seems to indicate that the system is resonate at that point.  However, despite the SWR figures, I'm wondering if I should shoot for 50 ohms of impedance at the center freq (27.185) which is the frequency most used (the whip antennas have an adjustable tuning stub at the top).  But then I wonder if having equal amounts of R and X should be avoided (as shown in the 26.965 reading).  Or should I be shooting for minimum phase angle?  Arrgghhh!!!

I could not get zero reactance (X) anywhere in the 11 meter band.  For grins I unscrewed both whips and found a zero reactance point of the co-phase harness (open circuit) at 33.065 MHz (R=21, X=1, Imp.=21).  With the above figures I made weak-but-readable contact with a base station 20 miles away on 27.255 Mhz, which, unless the base station had an impressive antenna system, I considered quite an accomplishment.  Any input from those familiar with using these meters or dealing with these figures is welcome.  Thanks.
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N8EMR
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« Reply #1 on: November 25, 2007, 07:25:43 PM »

I am a bit confused. If your talking CB co-phased whips then just slap them on  and get a good SWR and be done with it. Your CB not going to talk any further with or with out them. If your talking about the 857 and the co=phased whips then just slap them on and get done with it since you cant use the radio to transmit on CB and your RX will see little if any gain with whatever CB antenna you put on the rig.
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W3LK
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« Reply #2 on: November 25, 2007, 07:32:22 PM »

Ignoring the fact that "co-phasing" of helically-wound whips on a mobile installation is a marketing myth left over from the CB craze, it's not the amount of metal connected to the ground of the antenna, it's the amount of metal UNDER the antenna that matters. Mirror mounts are just about the worst mount for an HF antenna, no matter what your fellow truckers tell you. You simple cannot get enough separation between the antennas at HF frequencies to have any noticeable effect.

<< This is partly a learning project prior to tackling a Yaesu FT-857D installation in an 18 wheeler...I could not get zero reactance (X) anywhere in the 11 meter band.>>

If you are planning to use the 857D on 11m, DON'T. It is illegal to use amateur radios, of any power output, on 11m.

73,

Lon - W3LK
Naugatuck, Connecticut
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KB5TJ
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« Reply #3 on: November 25, 2007, 09:19:08 PM »

No, the FT-857 is not used in this project, I simply mentioned it in passing.

What I am trying to do is obtain the best possible match to the co-phased whips at CB freqs using a CB transceiver. Whether this is an efficient antenna system is irrelevant, since there is no efficient antenna system for this particular type of installation. These antennas are not mounted on the mirrors; they are mounted directy to the side of the aluminum cab, similar to what one would have if you mounted one whip on each side of a pickup truck bed that had a fiberglass camper shell.

What I learn from this project may be of help when I tackle the more demanding HF project. The MFJ-259B is giving me a lot of information which I am trying to analyze and understand.  If I merely 'slap something on' and hope that it works, then I have learned nothing.
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K0BG
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« Reply #4 on: November 26, 2007, 07:13:48 AM »

The efficiency of any 1/4 wave HF mobile antenna (CB included) is directly related to the image plane under it, not along side.

Co-phasing has zero effect on the image plane losses. In fact, you probably could establish a case where co-phasing actually increases the loss factor in this installation.

Worse, is the use of spirally wound antennas, and shortened ones at that! You have to remember, it isn't the amount of wire that comprises the antenna, it is the physical length of the radiator!

Length does matter. Within reason, doubling the length increases radiation resistance by a factor of four. All of this is for naught because the image plane losses are sky-rocket high.

Add in the very poor Q of the loading coils (most assuredly less than 20), and the efficiency is probably less than 5%, co-phasing notwithstanding.

Alan, KØBG
www.k0bg.com
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KB5TJ
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« Reply #5 on: November 26, 2007, 10:37:54 AM »

Alan, all your points are valid, but they are ones that I already understand.  The issue is making the best of a bad situation -- I have to live in the real world, and in this particular installation, the components outlined are the only ones that are practical.  5% efficiency is better than 2.5%.  If I had a practical location for a 108" whip, this job would have been completed quite some time ago ;-)

I am now making changes to the co-phase harness, for I believe it was incorrectly manufactured (wrong velocity factor).  At this point I have it at one-quarter wavelength, and am adjusting it to zero reactance at the operating frequency (including the antenna mounts), which also produces an SWR 'dip',  using the MFJ-259B.  The thought is that the antennas, once reinstalled, can then be adjusted for 'natural' resonance without them also having to compensate for line problems.  This approach may be overkill, or may even be wrong, but so far I've had no luck obtaining any useful technical guidance.
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KJ4AQT
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« Reply #6 on: November 26, 2007, 12:00:37 PM »

I Personally would not co-phase, imho the only reason to do so is to look cool, if you need matching antenna's then place a dummy on one side

what kind of truck is it?

on my 770 volvo i do not use the stock mirror mount or coax, that is asking for trouble, i got a seperate mirror bracket and mounted a wilson 2000 on the driver side and a 2 meter 5/8 wave hustler on the passenger side. also i removed both of the stock whips
as wll

On the 2 meter i can hit a repeater 20 miles away with good signal (50 Watt Kenwood Mobile)

on the cb side i can get 5 miles on the best day, but the 11 meter band is so full of garbage i dont expect much more running legal. and since i value my Amateur license more than 11 meters i will not run illegal
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KB5TJ
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Posts: 10




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« Reply #7 on: November 26, 2007, 03:05:17 PM »

Well, I did debate about tackling the co-phase arrangement, and here is my hypothesis, assuming a marginal ground plane:

If you mount a whip antenna on the left side of a vehicle, the omni pattern is going to be significantly distorted to one side.

If you mount a co-phased whip on the left and right side, less than 1/4 wave apart, while you will get no 'gain'/directivity, you will likely balance out the radiation pattern approximating a single center-mounted whip. With a decent match (which seems to be elusive), I would futher hypothesize that you may increase your receiver's reception somewhat, especially in mobile operation -- although with less than optimal spacing I have no idea if some type of phase imbalance would cancel that out.

The truck is a Freightliner Century.

I wound up getting the co-phase harness and mounts to resonate (reactance null) at the desired center freq with R=9 and X=0 (each leg of the harness 7'5", for a calculated and expected VF of 0.82), then screwed in the whips and tweaked them slightly (adjustable tips) to also resonate (SWR null) at Fc with resultant R=93, X=26, and Z=99. Final SWR @ Fc was 2.0:1, and given the Z=99 figure, that looks like a true value. At some point in the future I'll do some ground strapping of the aluminum cab to the frame and see if that improves things (it should).

Final numbers from MFJ-259B:

FREQ . / R ./ X ./ SWR / Z ./ Phase

26.965 / 76 / 53 / 2.5 / 93 / 35 deg.

27.185 / 93 / 28 / 2.1 / 97 / 15 deg.

27.405 / 40 / 43 / 2.8 / 59 / 46 deg.

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K0BG
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« Reply #8 on: November 26, 2007, 06:30:32 PM »

Mounting any antenna, on any vehicle, in any fashion, will to some extent, distort the pattern. What's misunderstood here is, the extent the pattern is distorted. While you might argue that the distorted pattern may (or may not) effect near field signals, any effect on far field signals is more reliant on the incoming wave angle.

As for co-phasing, let's be blunt; it is a crock of bovine fecal matter when you're dealing with HF frequencies on your average vehicle, and this includes semis.

While a 102 inch whip may provide better reception than a shortened spirally wound antenna, there are alternatives. In fact, if the cab is steel, or you can fashion some sort of image plane under it, a NMO27 mounted on the roof will out perform any spirally wound antenna, even if it is co-phased.

You have to keep in mind that verticals are half of a dipole. This is why they are sometimes referred to as monopoles. The missing half is the image plane. Co-phasing is not a substitute for an adequate image plane. Neither are mirror mounts.

Alan, KØBG
www.k0bg.com
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KB5TJ
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Posts: 10




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« Reply #9 on: November 26, 2007, 07:16:16 PM »

Alan, I've been fiddling with MF/HF/VHF transmitting antennas of various types for 40 years.  That's a long time, so I suppose it is possible that I've forgotten what a monopole is ;-)

In my particular case the *only* alternative was to mount one helically wound whip to the side of the cab, exactly where one of the co-phased whips now resides, and feed it directly with 50 ohm line.  That may have been as good or better than the approach that I undertook, but I partially undertook it as an educational exercise -- and on that point, the purpose thread was not to debate my approach, but to obtain assistance in interpreting the readings of the MFJ-259B and understanding co-phase harness theory in general.  

Unfortunately it has failed miserably in that endeavor.

73's
Mark
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KJ4AQT
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« Reply #10 on: November 27, 2007, 07:38:47 AM »

also on a century class the roof is fiberglass, as most trucks today

i have grounding braid and am thinking of grounding as much as i can to the frame which is the main piece of metal in  the truck, of course then add a 53' trailer and you have more ground plane
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K8KAS
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Posts: 569




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« Reply #11 on: December 03, 2007, 05:10:12 PM »

The problem in matching two short antennas and expecting 50 ohms, it will never happen. A 4 foot
heli wound whip would have a radiation resistance of 10 to 15 ohms, two of them connected would show 1/2 that or 5 to 7 ohms. What you need now is an auto transformer or some other type system to match the low RF impedance and give you a 50 ohm load. Now this is why co-phased antennas are total BS, you have two poor antennas to start with and then have to add more loss by reason of the matching network loss. The slight pattern compression (gain) will never make up for the poor antennas and the matching losses. A single 6 or 8 foot 3 KW Wilson or 3 KW Valor antenna and a 500 watt amp is the way to be noticed. Just kidding....
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KB1NCP
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Posts: 10




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« Reply #12 on: December 09, 2007, 10:30:04 PM »

Still no answers I see. Well some time ago I came across a good article in CQ magazine. It was on co-phased hamsticks on a little car. They were using 18 ft of 50 ohm coax to each antenna. This connected to a coaxial tee fitting and Then had a run from the branch to the radio. I recall the lengths were critical and the key was getting 2 MFJ-910 at the feedpoints of the antennas. I will find the magazine and give you the issue number, so you can look it up, or I can scan the article and email you if you provide a email address.
73 Bill KB1NCP
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