Mobile Cophased Dipoles

[StrangeBrew]

I would go with a standard phasing harness myself and use 50 ohm co-ax from the harness to the radio if more length is needed to reach.

I would also consider using identical antennas for both legs of the di-poles, maybe 102" whips for the top and bottom but put a bend in the bottom one so it runs forward about a foot or so off the ground. Hams bend the legs on di-poles all the time to fit available space.

 

[Sarasota Slim]

I would go with a standard phasing harness myself and use 50 ohm co-ax from the harness to the radio if more length is needed to reach.

I would also consider using identical antennas for both legs of the di-poles, maybe 102" whips for the top and bottom but put a bend in the bottom one so it runs forward about a foot or so off the ground. Hams bend the legs on di-poles all the time to fit available space.

Standard phasing harness and 50 ohm coax is pretty much what I had in mind.

In regard to bending the bottom half; that's a new thing to consider. I hadn't thought of that. But, how to hold the bend? I'll have to ponder that one.

Thank you for the ideas.

 

[w9cll]

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

The whole thing will not be efficient, you will have no counterpoise for the radiator to work off of. A true dipole needs equal parts. Bending the bottom whip is an option.

 

[Sarasota Slim]

The whole thing will not be efficient, you will have no counterpoise for the radiator to work off of. A true dipole needs equal parts.

Right; 1/4 wave on top, and 1/4 wave on bottom... which has been my intent, all along.

Strangebrew posted a suggestion, above, that I intend to investigate the feasibility of, but prior to that, I was still looking at 1/4 waves; an uncoiled, full length 1/4 wave for the top, and a compacted, shorter in physical length, 1/4 wave for the bottom. But, either way, still quarter waves on either end.

So, I guess what I'm not understanding is the lack of counterpoise, as counterpoise is the entire purpose of the bottom 1/4 wave antenna.

 

[w9cll]

So, I guess what I'm not understanding is the lack of counterpoise, as counterpoise is the entire purpose of the bottom 1/4 wave antenna.

Are you sure its 1/4 wave? Its a loaded antenna and who knows what it is. Anyway try and see what happens

 

[The DB]

Are you sure its 1/4 wave? Its a loaded antenna and who knows what it is. Anyway try and see what happens

I know what it is, it is an electrical 1/4 wavelength long, or very near (assuming their is some extra for tuning purposes). I'm not sure what your concern is over this, as long as it provides a resonant point near the end that attaches to the mount it should work fine.

Actually, the fact that a center fed dipole is known to have a resonant impedance higher than 50 ohms (72 ohms naturally), and the fact that mounting an antenna closer to the earth is known to raise that impedance even more, the loading coil might actually provide a benefit here as it is known to lower the feed point impedance. However, I doubt it will lower it enough to account for the above.

I am also curious about the concerns over efficiency. I'm not saying that their shouldn't be a concern over efficiency, but the potential areas of efficiency that have been mentioned will be minor to the cause of the most inefficiency in the antenna system. The system before the antennas won't have much more in the way of losses than the setup for a single antenna, assuming it is done right that is. Shortening an antenna from 8 or 9 feet to 5 feet will have some losses, sure, but not a drastic amount. Not anything like running a 40 meter ham radio antenna through such an antenna anyway...

The area I would be concerned about losses is putting the tip of a center fed (like) vertical dipole near the earth. Tips of antennas don't like being close to the earth. Getting the lower tip of such an antenna the first half wavelength in height above ground makes noticeably more of a difference than doing the same with a ground plane antenna. The earth below is the biggest enemy of an antenna system, their is a reason the phrase "height is might" exists. This also makes ground plane antennas the antennas of choice for mobile installations as their layout isn't affected as much by the presence of the earth being nearby.


The DB

 

[Robb]

There might be an easier way out of this dilemma to make a quasi-dipole work in a co-phase arrangement. But you will not be using 102" steel whips - simply because that idea is mechanically impractical as well - IMO.

Awhile back - when skip conditions were peaking - some guys were making a horizontal dipole with a pair of 5 ft firesticks with a 3/8"x24 truck mirror mount. Anyone know/remember the name of that thread? It was popular for awhile. Anyway, if you were to mount the same setup vertically on both sides of the truck with a co-phase harness; then that might work.

This would take care of the clearance problems for both ground clearance and maximum height clearance. Of course, this would also be a compromise of the better characteristics of the 102" whip/dipole. Which means it would not have nearly the same bandwidth (~2mhz/total) or gain as a true dipole would have (+2.2dbi gain); but its polar pattern would be similar - which is a gain in itself.

But I'm not sure the cost would be worth the experimental time/money/aggravation to get those 'ducks in a row' - IMO. Not saying this would be that easy.

 

[Riverman]

Awhile back - when skip conditions were peaking - some guys were making a horizontal dipole with a pair of 5 ft firesticks with a 3/8"x24 truck mirror mount. Anyone know/remember the name of that thread? It was popular for awhile. Anyway, if you were to mount the same setup vertically on both sides of the truck with a co-phase harness; then that might work.

Here's a pic of mine. Can't find the thread.

Worked great. Seems to me a co-phased set (vertical) on a truck would work fine as well.

 

[StrangeBrew]

Whatever is used it's going to be right next to the cab of the truck, it's never going to be an efficient system. No need to split hairs over what is by it's very nature a compromise.

When I was slip seating I had an antenna on a vice grip mount, I had a 108" wire connected to the mount that I would run in the window and scatter on the floor. It didn't work well but it did work, you'd be surprised at how half assed you can get and still get some signal out.

Now a perfect system would work best but that really isn't a possibility here, my example above may have worked but you could definitely do better. So just accept the fact that it won't be perfect and try to get it as close to the good end of the spectrum as is practical.


PS: I do agree with Robb that using shorter antennas top and bottom so the center could be mounted higher is something to consider just for simplicity's sake.

 

[Sarasota Slim]

Lots of good information, and stuff to ponder and consider; thank you all, for the well thought out responses and discussion.

I knew from the gitgo, that this project would be a compromise. It's my understanding that mobile antennas, in general, are a compromise. I'm OK with that (not much choice, but to be OK with that, lol).

I probably should have posted this thread BEFORE I bought the 102" whips. But, I just happened to run across them on a day that they were on my mind, so I bought 2, on a whim. They've been riding in my load lock rack, ever since. Over the past couple of years, I've spent hundreds of dollars on trying new antennas. I've already got the 102's, so I'll play with those, for now, but I'll be open to trying other things, too. (And, knowing me, I'll not only be open to it, but inclined to it, too. I just can't leave well enough alone. )

I don't mind the height of the antennas; I rather like the idea of taller ones. Most of the lower structures that I drive under, or through, are at lower speeds, anyway, so whacking the antennas on things doesn't much concern me. The last antenna that I had that high, survived (and is still in use, just about a foot lower, now.)

Using fiberglass for the top half, I'm a little shy of; I've broken a few. Winters are hard on antennas, with ice build up. The lower half is less a concern. I can tie those in place, so that they don't move much. It's also far quicker and easier to wipe them down with WD40, to discourage ice.

One thing with mounting antennas on the rear grab bars, is the grab bars are mounted to fiberglass ferrings, which get the antennas out about 20" from the corner of the metal truck walls, so while they may still be close (obviously), they're not pressed up against the body.

@StrangeBrew : Interesting that you brought up the vicegrip temp mount. I keep one of those, along with a spare radio, on my truck. Occasionally, the wife and I have to "rescue" an abandoned tractor, or drive a loaner for a couple of days. Since I've had such great luck with my current dipole setup, I was going to do something similar with the vicegrip. I was figuring on maybe a couple of Firestiks on fold downs, to make it easier to store, and quick to put up.

@Robb:

"But I'm not sure the cost would be worth the experimental time/money/aggravation to get those 'ducks in a row' - IMO. Not saying this would be that easy."

It comes out of my entertainment fund, lol. Also, it's a learning experience for me. And, there's not but so much that I can tinker with, while on a truck. I have to have SOMETHING to do, to occupy my hands.

 

[Needle Bender]

Lots of good information, and stuff to ponder and consider; thank you all, for the well thought out responses and discussion.

I knew from the gitgo, that this project would be a compromise. It's my understanding that mobile antennas, in general, are a compromise. I'm OK with that (not much choice, but to be OK with that, lol).

I probably should have posted this thread BEFORE I bought the 102" whips. But, I just happened to run across them on a day that they were on my mind, so I bought 2, on a whim. They've been riding in my load lock rack, ever since. Over the past couple of years, I've spent hundreds of dollars on trying new antennas. I've already got the 102's, so I'll play with those, for now, but I'll be open to trying other things, too. (And, knowing me, I'll not only be open to it, but inclined to it, too. I just can't leave well enough alone. )

I don't mind the height of the antennas; I rather like the idea of taller ones. Most of the lower structures that I drive under, or through, are at lower speeds, anyway, so whacking the antennas on things doesn't much concern me. The last antenna that I had that high, survived (and is still in use, just about a foot lower, now.)

Using fiberglass for the top half, I'm a little shy of; I've broken a few. Winters are hard on antennas, with ice build up. The lower half is less a concern. I can tie those in place, so that they don't move much. It's also far quicker and easier to wipe them down with WD40, to discourage ice.

One thing with mounting antennas on the rear grab bars, is the grab bars are mounted to fiberglass ferrings, which get the antennas out about 20" from the corner of the metal truck walls, so while they may still be close (obviously), they're not pressed up against the body.

@StrangeBrew : Interesting that you brought up the vicegrip temp mount. I keep one of those, along with a spare radio, on my truck. Occasionally, the wife and I have to "rescue" an abandoned tractor, or drive a loaner for a couple of days. Since I've had such great luck with my current dipole setup, I was going to do something similar with the vicegrip. I was figuring on maybe a couple of Firestiks on fold downs, to make it easier to store, and quick to put up.

@Robb:

"But I'm not sure the cost would be worth the experimental time/money/aggravation to get those 'ducks in a row' - IMO. Not saying this would be that easy."

It comes out of my entertainment fund, lol. Also, it's a learning experience for me. And, there's not but so much that I can tinker with, while on a truck. I have to have SOMETHING to do, to occupy my hands.

I LIKE IT!

I think 2 more 102" would do nicely, the top 3' or so of the top whips bent back at a 90° angle and the bottom 3' of the bottom whips also bent back at a 90°, leaving you with about an 11.5' height, 13.5' at the high point, 24" above the ground should work well,
plus,
it might appear more like a 120° dipole, lowering the impedance from a typical 74Ω for a 180° dipole to closer to the target 50Ω you're wanting.

...and 103" is actually a perfect 1/4 wave for the center of the CB band, and usually the mounting system adds about an inch to the radiating surface, leaving you needing, well what do you know, right about 102"! - Who'd a thot, Eh?

Now, for your future reference;

When you use an electrical 1/4 wave of 75Ω... (measuring the 1/4 wave in air or about 108", multiplied by the "VF" or "Velocity Factor" - a manufacturer's spec for all coax cable, usually 66% for most low-mid priced 50Ω coax, about 75% - 81% for 75Ω) ...in line with a 100Ω feed point, it reacts by providing a 50Ω reading at the other end, or 100Ω at the other end from a 50Ω - It "works both directions",
So,
Putting one electrical 1/4 wave of ( MY favorite: 81%VF RG-11) 75Ω at the 50Ω antenna mounting connector - going to one side of a coaxial T connector, the other antenna identically configured and to the other side of the T connector, will provide you TWO 100Ω loads on the top of the T connector
- in parallel,
so,
100Ω ÷ 2 = 50Ω at the bottom ot the T - YAY!!! Woo- Hoo! Yippie-Ky-Yay Mothe...
Well, you get the idea...

In case the 7 feet of 81%VF or so 75Ω, times 2 = ~14' isn't long enough to reach the T, you can place identical lengths of 50Ω from each antenna mount to it's respective 75Ω attached to it's side of the T.

I recommend using a length of 50Ω (from each antenna mount) which makes up the difference of 18' when subtracting the length of the 75Ω - so, if the 75Ω is exactly 7' 2" then I'd cut the identical lengths of 50Ω to 10' 10" so you end up with 18' total to the T and as short a length of 50Ω from the T to the radio/amp as possible, to help cut down on RFI.

You should end up with two ~50Ω antennas providing a single ~50Ω load to the radio/amp,
- NO NEED FOR A TUNER!

- Remember, tuners usually waste 10% -15% of your power, and a 2:1 SWR is only 10% loss in 50Ω coax, so who needs more gear, expense, connectors & wasted space?!?
- If 2:1 or less, Vaya con Dios!

Also, the highest current radiation from a 1/2 wave dipole is nearest the center so that's where your main energy lobe will be.
Conversely, you'll measure Highest Voltage at the ends, so depending on your power level those voltages could be harmful should someone decide to touch an antenna tip while you're transmitting
- Also, make sure the ends stay clear of any nearby metal to prevent arcing!

Anyway, I'd expect a rather high level of performance from your design creation - one which has entered my mind on many occasions.

I can't wait for your field report!

73 & happy DXing!

 

[Sarasota Slim]

@Needle Bender:

Yes, that might could work. Bending the top of the top antenna back by 3' would allow me to slide the mounting assembly up from the bottom of the grab bar, to the top. (If the power radiation is highest at the center, then moving the center higher up is good, yes?) Then doing the same to the bottom, I would have the whole thing a foot from the ground, so that would be good. However, the bottom antennas would then rub against the sidewalls of my front drive tires. Come to think of it, the top antennas, being bent back, may possibly arc on the top of the trailer. I don't know how much power there needs to be, before that's a concern, but I'm running a 100 watt amp. (At least, that's what RM claims the KL-203p is. It's not quite that much.) Why bend back, and not forward? If I bend the bottoms forward, I would have (or could possibly figure out ) some way to secure it, to keep it from smacking against the side of the truck (or rubbing against the side of a really expensive tire) If the top antennas were bent back, but maybe tilted forward, that might keep them off of the trailer.

The stuff for future reference... lotsa maths, lol. Sadly, I did not inherit my great grandfather's insane math skills. I'm going to have to read that a few more times so that it will sink in

With this, here:

"In case the 7 feet of 75%VF or so 75Ω, times 2 = ~13.5' isn't long enough to reach the T, you can place identical lengths of 50Ω from each antenna mount to it's respective 75Ω attached to it's side of the T."

Are you saying that I should have 7' feet of RG59 running from each antenna to a T? I'm guessing that you used the "velocity factor" formula that you mentioned, but I don't quite see the math. In the way of coax, I have a Firestik Fire-Flex K-9A co-phase harness (each lead being 18') that I could easily shorten, if need be. I don't know what the velocity factor is, though, and the website doesn't say, either. The website DID mention that the coax was 72ohm, as opposed to 75ohm, if that makes a difference.

I appreciate the insight

 

[M0GVZ]

Shortening an antenna from 8 or 9 feet to 5 feet will have some losses, sure, but not a drastic amount.

Actually it'll have a serious amount of loss, somewhere in the region of 75%. Radiation resistance is a function of the electrical length of an antenna and current flow over that is dependent on the Radiation Resistance. It works on a function of the square and directly affects efficiency so double the length gives four times the gain and reciprocally halving the length gives 1/4 of the efficiency.

I did a video on Youtube where I doubled the length of the whip on my Tarheel starting out at 0.8m then to 1.95m and finally to 3.2m. Each increase in length of the whip resulted in roughly 6dB of gain when receiving a beacon. Working backwards every time you halve the length you see roughly 6dB or 75% loss.

 

[The DB]

Actually it'll have a serious amount of loss, somewhere in the region of 75%. Radiation resistance is a function of the electrical length of an antenna and current flow over that is dependent on the Radiation Resistance. It works on a function of the square and directly affects efficiency so double the length gives four times the gain and reciprocally halving the length gives 1/4 of the efficiency.

I am well aware of radiation resistance and how it works, it isn't the radiation resistance, in and of itself, that is important for efficiency, but its ratio to all antenna losses added together. A radiation resistance of 2, and a total loss resistance of 4, will yield a 33% efficiency, and a radiation resistance of 1000 and a total loss resistance of 2000 will also yield a 33% efficiency (their is actually a bit more to it but this is enough to give a general idea about what I am talking about).

This formula you are referring to seems to be more a rule of thumb than anything. Simply changing the coil to one of a different Q can change the antenna's efficiency. Their are nice charts that show this in the 23'rd edition of the ARRL Antenna Book, one of them showing that in one case simply changing the Q of the coil (specifically from 450 to 150) can vary the efficiency for two antennas of the same length at the same frequency by over 15%. Where the load is also makes a difference, from the same chart, in one case where the load is along the length of the antenna can have as much as a 10% difference in efficiency as well. Does this formula of yours take these variances into account? These are just two examples.

How about real life experience, as some people swear by those, and i have seen this happen multiple times. A full length 1/4 wavelength CB antenna mounted to a vehicle's bumper and a five foot or so antenna mounted to the same vehicle's roof. The tips are near the same height. The five foot antenna mounted to the roof tunes to a stronger strength using a field strength meter, and I wouldn't call the different results particularly close either (opinion of someone that uses field strength meters regularly for tuning). If their were a 75% loss just from going from a full length 1/4 wave whip to a 5 foot CB antenna this wouldn't, and in fact couldn't happen irregardless of the different mounting positions. This is why I tell people to get the longest antenna that they can mount as high on their vehicle as possible (or sometimes as much metal under the antenna as possible)... In my experience, most of the time at CB frequencies this will get you best results.

I did a video on Youtube where I doubled the length of the whip on my Tarheel starting out at 0.8m then to 1.95m and finally to 3.2m. Each increase in length of the whip resulted in roughly 6dB of gain when receiving a beacon. Working backwards every time you halve the length you see roughly 6dB or 75% loss.

I've seen this video before and have just two comments about it. One, the longest antenna you used is electrically shorter on the frequency in question than the five foot CB antenna I mentioned. I wouldn't assume that switching to a frequency that is far closer to the resonant length of your longest antenna will yield as large a result between the longest two antennas. And Two, you specifically list a 6 to 7 dB difference based on an s-meter reading. That is likely not accurate as even ham radios s-meters are known to be far from accurate, some of them in some cases showing less than a 2 dB difference from one s-unit to another in some cases. All you can reliably tell by an s-meter is that one of the received signals is stronger than the other, you cannot tell by how much without first doing additional work.

Aside from the s-meter reading conversion to dB, it was a good video, and I would have no problem recommending it to people as an example of the difference of changing the antenna's length for an HF ham radio.


The DB

 

[rabbiporkchop]

For certain trucks, this is a great idea.
I did this with a Peterbilt 387 and it worked great. Mounting them outside the rear fairings will work best.
Plastic body trucks and plastic fairings are the best scenario to use this approach.