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Mobile Cophased Dipoles

Sarasota Slim

Sarasota Slim

Well-Known Member
Nov 18, 2016
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I want to run cophased dipole antennas on my truck. I'm currently running a single dipole, and have been mostly happy with the setup. My current antenna is mounted to the driver side mirror bracket. This leaves a "deaf spot" on the passenger side, rear of the truck, that I would like to eliminate. Also, for some weird reason, my antenna hums loudly, when I am driving at about 40mph. When I say it hums, I don't mean that a humming noise comes from my radio, I mean the actual antenna hums. It doesn't generally bother me, but it does annoy the wife.

So, what I want to do, is mount dipoles on the grab bars, on the rear of the tractor. Having flipped the grab bars, I can get the antennas out from behind the cab.

The antennas would be approximately 8 1/2 feet apart. It would take about 20 feet of coax to get from the radio to the driver side antenna, and, depending on how I run it, between 20-26 feet of coax for the passenger side.

I have 2 102" whips that I intend to use as the top half of the dipole, and will most likely use a 4' 1/4 wave fiberglass whip for the bottom half.

Also, in between the radio and antenna, there will be an impedance matcher, an amp, and an swr/pwr meter that I like to keep inline.

If anyone has any tips, tricks, suggestions, or advice, I am open to hearing them. I may even begrudgingly accept constructive criticism, lol :LOL:
 
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Sarasota Slim said:
The antennas would be approximately 8 1/2 feet apart. It would take about 20 feet of coax to get from the radio to the driver side antenna, and, depending on how I run it, between 20-26 feet of coax for the passenger side.
Click to expand...

When it comes to the matching device you mention, you need something that will convert the 50 ohm coax input to 100 ohms out. This should be placed right before the splitter, which will feed each side of the split 50 ohms. Then the lengths from the splitter to the antennas need to be identical, and the same type of coax, if not you won't have a cophased setup. This part is important.

This isn't how cophased antennas are normally done, but it will still work. Most people just use a cophaseing harness, which has its own limitations.


The DB
 
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If you're going to phase the antennas you will need a phasing harness. You will need to determine your desired phase angle, and cut the coax to the correct length. (Forced current approach) Yes, coax length will matter.
 
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How about the practical side of this? An 11m dipole will be close to 18 ft tall. How are you going to mount it on a truck?!? What is the standard height of a truck? Or - how much vertical clearance are you allowed? Curious.
 
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w9cll said:
Not going to be very efficient but will work. Your counter poise should be equal to the top half.
Click to expand...

Which part is inefficient? Coax length, antennas, or both? And why?

My current dipole setup consists of this antenna as the top half:

https://www.zoro.com/k40-antenna-op.../i/G0157888/?gclid=CPOBlevn89MCFY22wAodHDEFAQ

And this antenna as the bottom half:

https://www.zoro.com/k40-antenna-tunable-48inh-t400plusbk/i/G0157720/

Thus far, it's been the best thing that I have tried, in the way of antennas.
 
The DB said:
When it comes to the matching device you mention, you need something that will convert the 50 ohm coax input to 100 ohms out. This should be placed right before the splitter, which will feed each side of the split 50 ohms. Then the lengths from the splitter to the antennas need to be identical, and the same type of coax, if not you won't have a cophased setup. This part is important.

This isn't how cophased antennas are normally done, but it will still work. Most people just use a cophaseing harness, which has its own limitations.


The DB
Click to expand...

The matching device is one of these:

Amazon product ASIN B0042A332G
I'm currently using it; it does very well.

I know that the radio requires 50 ohm impedance from the coax, and I know that co-phased antennas want... what, 70? 70-something? Ohm impedance. (Still trying to figure the math behind that one) But, where is this 100 ohm coming from?

I will pay attention to my lengths from antennas to splitter. I had an idea that it may be important; that's now confirmed. Thank you.

Yes, I realize that I'm looking to do something a little abnormal, but there are limitations to overcome on modern trucks, especially when I don't own it, and will spend less than 2 years in it, before getting a new one.
 
trees said:
If you're going to phase the antennas you will need a phasing harness. You will need to determine your desired phase angle, and cut the coax to the correct length. (Forced current approach) Yes, coax length will matter.
Click to expand...

"Determine your desired phase angle"

Please pardon my ignorance; but, what does that mean?

"Forced current approach "

Again, what is that?

It reads like you just gave me some very sound advice, and I would like to take it into consideration, but I completely lack the education to know what you are talking about. I apologize, but could you please reiterate that, in layman's terms?

I DO have a phasing harness, I think.

https://jet.com/product/Firestik-K9...-with-Pl259s/21a2380f1b9442a1b5020aed108edd49

^^ Is that what you're talking about?

Thank you
 
Robb said:
How about the practical side of this? An 11m dipole will be close to 18 ft tall. How are you going to mount it on a truck?!? What is the standard height of a truck? Or - how much vertical clearance are you allowed? Curious.
Click to expand...

The bottom half of the antennas will be 1/4 wave, 4' fiberglass whips. The top half will be 102" steel whips, with rods added to make them 108". Mounted at the bottom of my grab bars, the antenna height will be just under 14.5 feet.

I would prefer the bottom half of the antenna to be at least 5' in length, but that would put the assembly too high, or have the antenna dragging the ground. So, 4' it is. Actually, if it WAS practical, I would love to have an entire 1/2 wave in length; but, it's not, obviously.

Standard truck height is 13' 6"; my antennas will be nearly a foot over that. I did have another antenna at that height, for a while. Aside from the fact that I was neither transmitting nor receiving (it was between cab and trailer), I had no problems. It did smack a few low overpasses, and a few low branches, but it survived. In fact, that antenna is now the top half of my current dipole.
 
I guess I just don't see the point of all that. Seems like a ton of trouble for no real improvement over a properly set up co-phased antenna system. Why try and re-invent the wheel?
 
9C1Driver said:
I guess I just don't see the point of all that. Seems like a ton of trouble for no real improvement over a properly set up co-phased antenna system. Why try and re-invent the wheel?
Click to expand...

The point? Well, I'll get to that, after I address the wheel, I guess.

But, first, I'll point out that I don't have a co-phased antenna system, currently. I did try that, once, on my 1st truck, which was, for all intents and purposes, identical to my current truck. The standard co-phased setup didn't work well for me. I couldn't get my SWR readings to drop below 2.5, and I didn't have much range. I tried single antennas. Had one on the driver side mirror mount, tried it on the passenger side mirror mount, and eventually tried it on the rear of the truck. That one was the worst, performance wise. Finally, I assembled the dipole setup that I am currently using, and I have found that it works the best, thus far. I receive more incoming signal than ever before, and I'm talking to people much farther away, than I used to. Only problem is that aforementioned deaf spot, where it seems that the truck is blocking the signal from reaching the other side of the truck.

So, then, on to the wheel. I'm not reinventing it; rather, the idea is to modify the wheel to better suit my purposes. (Which, by the way, in regard to actual wheels, is done quite regularly. )

The point, then. What is the point? Well, what's the point of any of this radio hobby? It's something to do, and one assumes that we derive some sort of pleasure from it. The point of this particular project, as I'm sure I've mentioned, is to attempt to eliminate this deaf spot that I have on the passenger side of my truck. I have already determined a single dipole to be superior to the standard method of using the vehicle as a ground plane. I understand that it can be done, however it requires doing a lot of stuff, fairly permanent in nature, to a truck that I do not own, and will occupy for less than 2 years. The single dipole turned out to work better, and set up with far less expenditure in time and effort. So, now I want to see what co-phased dipoles will do.

Plus, I am a compulsive tinkerer. I have to modify everything. It drives the wife nuts. I just can't leave anything be. And, I tend to go about things in slightly different manners, than others would.

Finally, if I didn't have this sort of thing to occupy my brain, sheer boredom and my compulsive tinkering obsession would probably cause me to start disassembling the pumps, every time I stopped for fuel. They'd probably put me in jail for that, so this keeps me out of trouble. :rolleyes::D
 
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