New ideas for 2 element mobiles!

[Shockwave]

I've long thought that most mobile 2 antenna beam type installations were not making the best use of the vehicles ground plane to maximize gain. The typical method of making the back antenna into a reflector and mounting the driven element in front, takes away from the constructive forward directional effect offered by having the driven element towards the back of the vehicle.

I decided to do some simple modeling in EZNEC+ and it turns out that adding a reflector to make a 2 element beam may indeed be backwards thinking in this application. There seems to be virtually no difference in the amount of forward gain offered between adding one reflector or adding one director. That is until you take into consideration the effects of the vehicles ground plane.

Keeping the driven element in the back provides the standard forward gain we typically see on vans or similar long vehicles. Therefore, adding a director rather then a reflector will reinforce the already existing forward gain. This comes at the expense of some backside rejection and is why we never see 2 element designs with a director. However, who cares about rejection if it's able to increase forward gain in this case?

I suspect this would only be beneficial in 2 element systems since adding 2 directors will not add as much gain as adding one director and one reflector in the case of a 3 element design. Sure could give someone the edge in any 2 element competition though! Any feedback on these ideas are welcome.

 

[Henry HPSD]

Hello Shockwave,

I havnt checked into "things",

But sounds to me you are on to something.

wasnt the max gain a 2el can produce with a director/radiator combination?
Looking at the "body" of the car we know if we place the mobile antenna at the back of the car the maximum field goes forward.

With the director/radiator setup you have and the advantage of the "body" and the max gain combination.

If maximum gain in a forward direction is what one is looking for then it sounds to me like a good plan to futher investigate...if only we had some more time lol.

You migth consider too look into "c-pole" antenna aswell..perhaps thats the way to go.

Kind regards,

Henry www.dx-antennas.com

 

[W5LZ]

I hate to 'rain' on your parade, but neither of these ideas is 'new'. That "C-pole" type antenna has been around in one form or another for at least 30 years.
If you take a look at a few of the VHF/UHF antenna arrays for direction finding I think you may recognize the 'director' + 'driven' elements in a few of them. Why the VHF/UHF antennas? Because they are much easier to deal with, more common, for mobile operations unlike HF antennas.
Still want to 'play' with the idea? Have as much fun doing so as you can stand! Nothing wrong with that. You might take that 'look' first though, I can save a lot of guessing about things.
- 'Doc

 

[jazzsinger]

I hate to 'rain' on your parade, but neither of these ideas is 'new'. That "C-pole" type antenna has been around in one form or another for at least 30 years.
If you take a look at a few of the VHF/UHF antenna arrays for direction finding I think you may recognize the 'director' + 'driven' elements in a few of them. Why the VHF/UHF antennas? Because they are much easier to deal with, more common, for mobile operations unlike HF antennas.
Still want to 'play' with the idea? Have as much fun doing so as you can stand! Nothing wrong with that. You might take that 'look' first though, I can save a lot of guessing about things.
- 'Doc

The C pole antenna is a non counterpoise antenna, whereas what Shockwave and Henry are talking about is a driven/director 2 element mobile that uses a counterpoise.

Using a reflector that improves rear rejection is a bit pointless on a mobile that can be rotated to any plane, whereas forward gain from a director makes much more sense.

 

[1iwilly]

how about the spacing we know that 6ft is perfect for 2 straight sticks and anything over 6ft to 86' is the key point in gain. nowmy question to you is when we add a 3rd antenna the director ends up longer than the driven element and the 3rd antenna is just a floater nothing gets connected to it
in doing it your way how would the director be mounted.??

I've long thought that most mobile 2 antenna beam type installations were not making the best use of the vehicles ground plane to maximize gain. The typical method of making the back antenna into a reflector and mounting the driven element in front, takes away from the constructive forward directional effect offered by having the driven element towards the back of the vehicle.

I decided to do some simple modeling in EZNEC+ and it turns out that adding a reflector to make a 2 element beam may indeed be backwards thinking in this application. There seems to be virtually no difference in the amount of forward gain offered between adding one reflector or adding one director. That is until you take into consideration the effects of the vehicles ground plane.

Keeping the driven element in the back provides the standard forward gain we typically see on vans or similar long vehicles. Therefore, adding a director rather then a reflector will reinforce the already existing forward gain. This comes at the expense of some backside rejection and is why we never see 2 element designs with a director. However, who cares about rejection if it's able to increase forward gain in this case?

I suspect this would only be beneficial in 2 element systems since adding 2 directors will not add as much gain as adding one director and one reflector in the case of a 3 element design. Sure could give someone the edge in any 2 element competition though! Any feedback on these ideas are welcome.

 

[gamegetter]

Therefore, adding a director rather then a reflector will reinforce the already existing forward gain.
QUOTE]

greetings

do you have any numbers on how much the reinforcement may be? thanks in advance

 

[W5LZ]

jazzsinger,
If you end up with the same reslts, what's the difference, the reflector/director thingy? I can imagine instances where not adding a 'longer' element would be nice though.

There are many antennas that don't require a "groundplane". That 'C'-pole being a loop is just another of those types of antennas. The biggest difference being where the feed point is. There was a similar such antenna in the ARRL's Hand Book during the '80s. No idea if it's still encluded in the later editions.
- 'Doc

 

[1iwilly]

you know this almost sounds like the 2 hot set up when you ende up with rear
as driven and front director. then if you want you can makea bracket and add a reflector but been that it is not set up as what they can end fire
i still want to know how would the front director be set up.??

 

[Shockwave]

Henry, In my simple model I was unable to notice any difference in forward gain between 1 director or 1 reflector once you take the vehicle ground plane out of the equation. The only significant difference seems to be a noted reduction in front to back rejection with the single director.

W5LZ, Jazz is right on this one. We very much want that front element to rely on some type of counterpoise and then add the most asymmetrical counterpoise we can to boost forward gain. If you're not following the point here, take all of the radials off of any ground plane antenna except for 1. You will notice a slight increase in impedance and a larger gain increase in the direction of the one radial at the expense of gain in all other directions.

Willy, Spacing is not nearly as critical as tuning the parasitic elements (director or reflector) lengths for maximum forward gain. The biggest change regarding spacing is its effect on impedance. When you move the spacing closer then 4.5 feet, you will see the impedance start to drop off rather quickly. More spacing usually increases gain but the change is very very small once you're over 5.5 feet. Therefore, in this case it's probably wiser to use the spacing to help adjust your VSWR. I recommend making the spacing adjustable by placing the driven element on a quality magnet mount for temporary testing. Extra parasitic elements can be tested the same way if you removed the coax from the mag mount and short the whip to ground by removing the insulator in the connector.

Gamegetter, determining the "already existing gain" of the vehicle ground plane with the driven element in the back varies quite widely from one vehicle to another. My rough estimate would be about 2 db. Without the variable of the vehicle ground plane, adding either a director or a reflector would give about 5 db forward gain. If one director element is added to a van with the driven element in the back, I estimate you could see up to 7 db if done correctly.

 

[Shockwave]

Idea for 3 elements only.

how about the spacing we know that 6ft is perfect for 2 straight sticks and anything over 6ft to 86' is the key point in gain. nowmy question to you is when we add a 3rd antenna the director ends up longer than the driven element and the 3rd antenna is just a floater nothing gets connected to it
in doing it your way how would the director be mounted.??

Willie, My idea for a 2 element antenna in this post is entirely different then what I think about 3 element mobile designs. I've seen many people make different attempts at getting a mobile director element to work in a 3 element configuration. Insulating the director from the vehicle ground plane by extending it out in front of it, leaves you with half of a director element. This is why your director element is longer then your other elements. Building a metal bracket that is RF bonded to the vehicle places the reflective ground plane behind the director. This causes the radiation currents in this element to favor the rear direction. Exactly what we don't want for a director.

What you should do is fabricate an isulative support out in front of the vehicle to hold a mirror mount bracket. Use the four bolts on the mirror mount to hold it to the insulated support with the notch that would normally mount to the tube on the mirror, facing forward. Remove the plastic washer in the aluminum mirror mount connector so that the top 3/8 thread is shorted to the metal below it.

Then insert a piece of horizontal aluminum tubing about 1/4 wavelength long, inside the mirror mount tube base so that it extends out in front of the director and the vehicle horizontally. Then bend this element down towards a 45 degree angle to improve the VSWR. This lower angled counterpoise on the director will pull the radiation currents in the forward direction. The length of the top and bottom angle half of the director must be equal and should be adjusted for maximum forward gain.

 

[1iwilly]

ok my question is i have no problem testing on my astro van i'm running 6ft 8'spacing but i honestly think 6 ft would be best for 2 straight sticks i'm thinking very hard if i want to drill another hole for 6ft as i already have a dummy plate in the front at 86' spacing but what i need to know is the director how does it get connected.?? does it just sits there on the ball mount not connected?? i have a mfj259 b so tunning should take long because if it gets grounded then it would end longer than the driven one no.???

 

[Shockwave]

ok my question is i have no problem testing on my astro van i'm running 6ft 8'spacing but i honestly think 6 ft would be best for 2 straight sticks i'm thinking very hard if i want to drill another hole for 6ft as i already have a dummy plate in the front at 86' spacing but what i need to know is the director how does it get connected.?? does it just sits there on the ball mount not connected?? i have a mfj259 b so tunning should take long because if it gets grounded then it would end longer than the driven one no.???

Eight inches in this case won't make much difference. Forward gain will not change to where you could notice it. Here is how to construct a director for a 3 element system where the director will hang out in front on the vehicle. Fabricate some type of bracket (preferably non conductive) that will hold a mirror mount type antenna bracket out in front of your driven element and at the same base height.

The idea is to make a sort of 1/4 wave ground plane antenna out of this mirror mount but it will only have one radial sloping downwards out in front. The top whip and bottom radial should be connected by removing the plastic insulator around the connector. You use the four bolts on the mirror mount to bolt it to your insulating support on your truck. If the support is conductive, you might want to insulate the aluminum mirror mount from the support and vehicle ground plane.

The only reason your director is longer then your other elements now is because it's floating and is only half an element to begin with. You do not tune the director with an MFJ. You keep the top and bottom halves equal lengths and adjust them both for maximum forward gain. I like to use a distant stable carrier in my receiver with a digital volt meter connected across its meter to increase the resolution.

 

[1iwilly]

you got me lost i need like arought drawing of the this mirror braket set up
and the part where you don't tune the element don't make sense if you don't tune it swr will go up so you would have to tune it for the lowest swr then adjust for gain.

Eight inches in this case won't make much difference. Forward gain will not change to where you could notice it. Here is how to construct a director for a 3 element system where the director will hang out in front on the vehicle. Fabricate some type of bracket (preferably non conductive) that will hold a mirror mount type antenna bracket out in front of your driven element and at the same base height.

The idea is to make a sort of 1/4 wave ground plane antenna out of this mirror mount but it will only have one radial sloping downwards out in front. The top whip and bottom radial should be connected by removing the plastic insulator around the connector. You use the four bolts on the mirror mount to bolt it to your insulating support on your truck. If the support is conductive, you might want to insulate the aluminum mirror mount from the support and vehicle ground plane.

The only reason your director is longer then your other elements now is because it's floating and is only half an element to begin with. You do not tune the director with an MFJ. You keep the top and bottom halves equal lengths and adjust them both for maximum forward gain. I like to use a distant stable carrier in my receiver with a digital volt meter connected across its meter to increase the resolution.