Placement of Radio Antenna on FB

[Richard Dziak]

I am adding a Racing Electronics Motorola CP200 radio system to the car.

I have had great input and opinions from many fellow racers here on ApexSpeed.

I will use a sheet of copper 5 mil foil under the antenna, 12" x 6" as a ground plane.

My questions:


Where's the best area on the car to place the antenna?

One person suggested in front of the shock cover which is rather low.

Any suggestions from those that have radios in their cars?

Has anybody used the 3db gain Shark Fin type antenna vs. the 6" whip style?

[reidhazelton]

Take this with a grain of salt, but I don't think it is all that critical. I have kenwoods, and have the antenna mounted on the shock cover with no problems. It is a little hassle when you have to take the nose off and it is tethered to the car by the cable. An eager crew member took the car cover off one morning and took off the antenna too, so rather than rush to get it fixed we just used the thread-on one that is on the radio itself, and even that worked. It was at Road America and I could hear them the whole way around. Radio is mounted above the bias adjuster.

For the ultimate reception, glue it to the top of your helmet...show the comp you mean business when it comes to your radios!

[rickb99]

I will use a sheet of copper 5 mil foil under the antenna, 12" x 6" as a ground plane.

My questions:

Where's the best area on the car to place the antenna?

One person suggested in front of the shock cover which is rather low.

Has anybody used the 3db gain Shark Fin type antenna vs. the 6" whip style?

Richard,
The Motorolas come in 2 flavors. VHF (around 150 MHz) and UHF (around 430 MHz). Which set do you have?

As the 'ideal' ground plane is a quater wave length. The VHF radios would like to see about an 18" quarter wave ground plane. If you have the UHF (probably) the quarter wave ground plane is around 6 or 7 inches. Barring getting the "ideal" bigger is better but try maintain the circularity as close as possible.

If would be 'better' to cut a circular piece of the copper foil. By using an asymetrical ground plane you could introduce some 'directionality' into the transmitted RF from the car (i.e. stronger RF signals going out in north/south or in the east/west directions.

As hyper-active as the F1 teams are about aerodynamics, there's a reason why they are still running around with monopole whip antennas on the front cockpit surround. They work and are not near the engine compartment!

I've seen shark fins but have no idea HOW they are designed. Tried to find info about them but couldn't get any design theory. Found an IEEE paper on theory but I'm not a member any more. There are only so many ways to get gain in an antenna. Use a log periodic, center feed, cicular polorization (needs double the transmitter power to radiate the equivalent RF in the V and H planes and a CP recieving antenna) or multi-element array.

Shark fins that are log periodic might be a good deal on ovals because they are directioinal by nature (if that's what shark fins are with 3dB gain) but not good on road courses.

Stick with the omni-directional whip, check your radio frequency and cut the ground plane in a 'close to' circular pattern if possible.

AS REID SAID:
Is this all worth it? Maybe, maybe not. Just depends on what tracks you run and what gets in the way. Hearing all the way around RA with just the whips is impressive but not a guarantee it will happen ALL the time or every where.

Do not glue your radio to your helmet with JB Weld

[Richard Dziak]

The radios that came in the setup were:

Motorola CP200 AAH50QDC9AA1AN - 4W, 4C UHF(403-440 MHz) Two Way Radio w/2250 mAh Li-ion & w/90-Min Charger. They sell at about $312.00 to $339.00 each. MSRP is $465.99 each.

I will cut the ground plane into a circle as you stated. The antenna provided was the 1/4 wave 6" to 7" whip antenna.

Most of the tracks I run are under 2 miles total length and always visible to the pit crew. The tracks are not like a Road America or Road Atlanta.

Down here in the deep Southwest, the land and tracks are pretty flat. We don't have trees to stand in the way.

Antenna on helmet sounds novel, and I am certain I will not attempt that one. It was a humorous idea however.

[Rick Iverson]

As the 'ideal' ground plane is a quater wave length. The VHF radios would like to see about 18". If you have the UHF (probably) the quarter wave ground plane is around 6 or 7 inches. Barring getting the "ideal" bigger is better but try maintain the circularity as close as possible.

Here!

[Stan Clayton]

I am not an EE, but I did do HF radio for a number of years, where signal-to-noise ratio, and consequently antenna configuration, is critical to successful operation, so maybe I can help a bit.

First of all, the term "ground plane" is a bit of a misnomer with respect to modern radios. The plane does not actually form an electrical ground. That function is generally performed by the outer foil sheathing in the antenna wire, whether the antenna is remotely mounted in a car with a 3' to 6' antenna lead, or the super-short lead to the rubber ducky in the radio body itself. That's why the foil ground plane in race cars often "float" with reference to the chassis...they aren't performing any electrical ground function, and completing a circuit to the chassis potentially exposes the radio and its signal to extraneous electrical noise. (Hint: don't electrically ground the "ground plane" to the chassis.)

The ground plane is actually a reflector that forces more of the RF signal to propagate outward and upward than if the plane is not present. Picture it this way...a stereo speaker faces you sitting on the floor a few feet out in front of a solid concrete wall. It will project more music (sound) at you than if the same speaker is out in the middle of an open field because the wall is reflecting the sound towards you that would otherwise propagate in the wrong direction. The foil ground plane in a race car performs the same reflective function for the radio's RF signal as the wall does for the speaker's sound waves.

Second, as noted above the size of the ground plane is important for maximum antenna performance, and the ideal part-wavelength ground plane should have a radius of an even divisor of the wavelength (one, one-half, one-quarter, etc.). Radios that operate at 403-430 MHz have wavelengths of 27.5" to 29.3". A ground plane selected for the midpoint frequency will give you the 90% solution for the radio's useful frequency range, so for such a narrow operating bandwith there is no real need to get closer than that in a practical sense. The midpoint of 403-430 is 416.5 MHz, so I recommend you use that frequency to set the diameter of your ground plane. A quarter wavelength ground plane for 416.5 MHz will have a radius of about 7.1". That's 14.2" across, which may be too wide to fit in the shock cover. Therefore, reducing it to one-eighth wavelength, or 7.1" in diameter may be required to fit the cover.

As with everything else in the race car, setting up the ground plane is a compromise, but if you follow the suggestions above you will be on the right track. Have fun!

Stan

[Richard Dziak]

Stan, Rick:

I have the 1/4 wave antenna which is about 6" to 7 " tall, a whip. I ordered a few sheets of 5 mil copper foil and will trim the foil into a circle and will make sure that the brass fitting on the antenna touches and secures to the round foil. I plan to cut a 6 inch diamater foil 5 mil copper ground plane. I placed the antenna on the removable shock cover and will attach the car radio to the inside of the shock cover where their is plenty of room. I will shorten the anntenna cable from the radio to the antenna with about 2 feet of antenna cable. I will however have to find a suitable volume level as once the radio is placed inside the shock cover, I will not be able to adjust the volume from the drivers arm length. Some sound level testing will be needed. I will attach the radio to the shock cover using industrial strength Velco and it will hold the radio fine and in place. The radio will be removeable for charging. All I will have to do is lift the shock cover, adhere the radio, connect the antenna and the radio harness to my helmet. About a 2-3 minute process.

I will send some photos once I have the system installed. I am awaiting the delivery of the copper foil sheets.

Thanks so much for the input. The wealth of knowledge here on ApexSpeed and the willingness to help and assist other racers is truley remarkable, thus making ApexSpeed the leader in knowledge and information to the formula race car community, both in the USA and world wide.

[rickb99]

TIME OUT, TIME OUT ! !

Richard,

I perhaps was NOT clear up above. In the other thread, I was a little more clear.

The idea is to get a QUARTER WAVE ground plane around the antenna. With your 400Mhz radios, that means you want the antenna centered on a 12" close to circular ground plane. In othere words, 6" to 7" in ALL directions.

The objective is to present at least a "quarter wave" ground to the antenna around its circumfrence. If you need to over laps sheets and cut an approximate circle that's fine. Just solder the sheets together, no problem.

STAN'S COMMENT about using a 1/8th wave ground plane if things are tight is INDEED true. But better if you can get the quarter wave in there though. RF propagation effects Stan described are what you are looking for.

Sorry if I caused confusion up above.

IMPORTANT, VERY IMPORTANT:

I do NOT recommend cutting and shortening the cable from the radio to the antenna. If the radio manufacturer did his job right, the VSWR of that cable is tested to give proper matching of the radio/cable/antenna.

If you cut the cable and put the connector back on, you won't know what the VSWR is and that could mess up both transmission and reception.

BETTER to put the radio in a more convenient location and "meander" the cable along the side of the cockpit with tie wraps or whatever being SURE that the cable does not cross over itself with a little (like 1/2") separation as it meanders along. This avoids cross coupling which could reduce the signal strength. Yes, the cable is shielded but there can still be some RF leakage.

IF you've already cut the cable (I hope not ) take it to a radio or tv repair shop and ask them to check the VSWR from 400 to 440MHz. If none around you, see if your local tracks communications guy has a VSWR meter to test it. If the comm guy can't do the 400 to 430 at least if he checks on their 400 Mhz comm. channel you'll have some idea if it's okay.

[Richard Dziak]

Rick:

I didn't cut the cable yet, so your post was just in the knick of time.

Hey, thanks so much for the input. It really helps. I just want to make sure that I get some good reception after installing the system. I have read were many drivers end up with sub standard reception and get frustrated. I appreciate your input, Stan's input. You took a technical situation and made it somewhat simple to understand. My business is finance, securities and economics, and some of the high tech stuff I read is difficult to understand. I understand money but not radio frequencies.

After your post I will mount the radio and use the orignal antenna cable and have the radio right next to my left leg, attached to the aluminum side panel inside the cockpit. Then I can make volume and channel adjustments with ease. This way I use the entire antenna cable.

Thanks again for all your wisdom.

[rickb99]

Well, Richard, you may do all this good stuff and STILL have a problem. But we'll work on whatever it is

[Richard Dziak]

Can the antenna cable be cut and have two disconnect connectors (BNC connectors) to remove the shock cover?

The cable length would remain the same but would give me the opportunity to remove the shock cover easily.

[rickb99]

???

Isn't there a BNC connector to attach the cable to the antenna? Does the way you're mounting it prevent you from partially removing it then disconnecting the antenna?

The reason is, no, not a good idea to just cut the cable and put another connector in the line. Again, it's the VSWR thing

VSWR:
Voltage Standing Wave Ratio. Making this simple.

It's sorta like a funnel. You put stuff in but when it hits the neck of the funnel it slows down. At RF frequencies the signal doesn't like anything in its path to RESIST it's attempts to get where it's going. If it encounters something resisting its flow it's called a high VSWR, some of the signal gets reflected backwards and never makes it to the antenna. The same thing can happen for the much smaller INCOMING signals if they encounter a high VSWR.

Whenever you put a connector in the cable, you add to the 'inherent' VSWR of the system. Worst case (happens rarely) you can end up with a serious mis-match and cause all kinds of problems.

Having said THAT. If you could find a shop to check the cable VSWR at your radios frequency, they could install another connector in the cable and verify that it's good.

Is it a sure thing the additional connector will mess it up? NO
Is it a sure thing the additional connector will not mess it up? NO
But if you don't measure it you don't know.

On the other hand I don't think a second cable would cost much. In fact, if you went to the company you purchased the radios from and asked them, they might be able to provide a built and checked 2 piece cable.

Am I being fussy about nothing? Maybe. But then when you have the next problem, a split, untested cable would be the FIRST thing to worry about.

[Richard Dziak]

Good points. I will leave the cable one piece until I can find someone to check the VSWR.

I will put the radios to a test once I get my helmet kit and install that. Hopefully next weekend.