elements on a 4 door sedan

[bigred222]

I would like to do a science project of sorts. I have a 1998 Audi A4 sedan. I currently have a wilson 1000 magnet mount on the roof of my car and a 102'' ss whip on my trunk. the swr is very good 1.3-1.1
Now what i would like to do is set it up with more elements.
I could really care less about asthetics. I just want to get the most possible gain i can, to make the vehicle talk as best as possible, what i plan to do is weld up some metal on to the back of my car, maybe using box steel stock to mount additional elements to the back of the vehicle.
what i would like to know is what the spacing should be?
Should I change the hot element from my wilson and use the 102 as a "hot" element?
Should I consider mounting elements to the front off the front bumper. I would like to get the most gain i can with the addition of antennas.
should. i scrap that idea and buy something like a 4 element maco and make a mobile mount for that so i can set it up when i go to the beach to transmit??
comments, help, ideas, any positive input would be greatly appreciated.

 

[loosecannon]

hi Bigred welcome back to the forum.

what you are doing is basically building a vertical yagi beam antenna, or at least half of one.

it will yield some additional gain and directivity over a single antenna, but it will never work as well as a 4 element beam mounted on a mast will.

what i would do is study up on yagi beam design, and use those specs to start the experiment.

also, over at mauldroppers dot com, those guys seem to be more into the competition type setups and may have some input for you.

good luck,
Lc

 

[Shockwave]

Red, there are many things you can do in this area. You first have to decide if you want to go through the trouble of setting up a horizontal beam every time you talk or if you just want to park and screw a few whips onto the mounts of your vehicle? Finding good places to set up an elaborate antenna and talk in this area is becoming harder. Most of the "big guns" are getting their radios done by Raccoon with the MOSFET mod and he's putting some of the dirtiest radios in front of the biggest amplifiers. Now if you try and park anywhere along the waterfront, you'll have the police or coastguard on your case just over the sight of the antennas. People have been wiping their radios out when they park and talk for so long, they use the preemptive strike method now.

Back to the original question. The flat sided beam you see most people set up with the mobiles is not really even generating a flat signal since they have it so close to the ground. Unless you can get it up over the typical 15 to 20 foot mark, the signal will not be close to fully horizontal. At 20 feet I found much better results using a 2 element quad than a 3 element Yagi. The difference was close to 2db. I made a portable 2 element Quad for a "Deuce and a quarters" (225) mobile application about 10 years ago. I bet if you ask him, he'll sell it to you since he does not use the mobile anymore.

If you want to go with whips on the vehicle, it's not easy to get right since every vehicle is different and presents different groundplane characteristics to the antennas mounted above it. You can't just copy what worked for someone else unless they had the same ride as you and that assumes they got it right in the first place. The first thing you need to know is the length of the vehicle from bumper to bumper. That will determine the amount of elements you can put up without adding extension booms. If you add booms, they should only be used on the directors. The reflector should be mounted all the way on the back of the roof.

The reflector should be cut for the correct calculated length and grounded through a variable capacitor with an upper range around 500 pf. Once the system is installed this variable cap will be tuned for maximum rejection. That usually happens around 300 pf. The driven element should be cut using the same method with a small coil across its input to ground. The coil will be adjusted for minimum VSWR rather than altering the calculated lengths of the elements. If there is room to add a director on the roof, great. If not you might consider adding the boom.

If you add a director on a boom, don't bother to try and tie it into the vehicle groundplane. The antenna is already too far out in front and above it to do much good. The fact the groundplane will be behind this element means any radiation currents here will favor the rear of the vehicle. Not good. It's better to add a separate counterpoise directly under the element. This does present ground clearance problems that can be handled two ways. First, you can load the counterpoise to reduce its physical length. Second, you can angle it forward and that will improve its forward gain too.

If you have some questions I may be able to give you a hand with this. There are many things that could be done to improve these types of antenna systems that are overlooked by most people. Then you see people like Sasha show up in that white van with antennas that no one's seen before, beat everything on the line. All he really did was take two 3 element antennas and figure out how to mount, phase and tune them correctly on a van.