DIY - Removing Your Radio Antenna

[TheDon]

Originally Posted by WTFM8

Well, in that case, one could simply scavenge a static, one-piece antenna and eliminate the need for the gearbox and other electrics associated with that most decadent of luxury features: the automatic mast antenna!

Some of the older Mercedes's came with a manual crank antenna... uber rare but they are out there. I like my antenna, it retracts

[ttoyoda]

I'm sure you all enthralled, but let me bring this back to hypermiling and your antenna, and the subject of using the defroster grid.

Well, I for one AM enthralled. Great post.

If we wanted fluff and krep, we would be off watching Oprah or Rosie O'Donnell all day while swilling a gallon of martinis out of plastic cups, not reading and posting here.

[Warhawk626]

Originally Posted by JJW

Note: skip to the "short version" for the payoff of my rambling!

Maybe this is an opportunity to help with the antenna, as I've been an amateur radio operator since I was 16, and I'm the son of an RF engineer. There is nothing special about car radio antennas, they are as they appear - a piece of wire hanging out there in the breeze. The wire does have two important properties - Its length will be a certain fraction of the free-space wavelength of the signal you intend to receive, and it is electrically insulated from the ground plane (which in this case is the body of the car). Its length is engineered so that its impedance will match the design impedance of the radio. In practice, you can construct circuitry to match any length you like to that impedance, but its cheapest and easiest for the manufacturers to simply make them the length that matches. Most older cars seem to have an antenna designed to be 1/4 wavelength on the FM band, which presents about a 75 ohm impedance (depending on the ground plane, the design could be anywhere from 34 - 90 ohms impedance). This matches to the 75 ohm coax and the input of the radio, making the energy transfer from the antenna to the radio optimal (mismatch causes loss). In case you are curious, the wavelength in feet is equal to the speed of light in feet per second over the frequency in hertz. That would make a full wave on the AM band (I used 1200) 820 feet, and a full wave on the FM band (I used 102) 9.6 feet. To be a 1/4 wave at the FM frequency, the antenna would be about 2.4 feet long. It suffices to say that cut for anywhere in the FM band, the antenna will be between 2.2 and 2.5 feet long as a car body isn't a perfect ground plane, so the dimensions could shift somewhat. That presents the problem that the antenna is very very short compared to the AM band wavelength, which means it will not present the same 75 ohm impedance on that band to the radio. There is usually some matching circuitry in the radio to match the very high impedance that the antenna would be on the AM band to the receiver. Some cars run preamplifiers and matching circuits outboard of the radio, and it seems to be very common with the short stubby antennas that are so common now.

I'm sure you all enthralled, but let me bring this back to hypermiling and your antenna, and the subject of using the defroster grid. All the radio really cares about is the impedance of your antenna. If the impedance right, at the size of our cars, the only other real variable is size. The bigger the better, up to a point (for reasons that would be equally as boring to you). Antennas can be thought of as having two halves.. I'm sure you've seen this on old TV rabbit ears. With the usual car antenna, the antenna makes up half of the system, and the body of the car makes up the other half. Clever eh? Radio waves don't care what it looks like. SO, when you see antennas in windows, you'll note that they have two halves.. they aren't using the body as another half.

A 1/2 wave dipole will have a feed impedance of about 50 ohms in free space. What's a halfwave dipole? It's like two antenna elements in a straight line facing away from each other and fed in the center. Each element is a 1/4 wave, so the total size of the antenna is 1/2 wavelength. Think TV rabbit ears spread as far apart as possible, so that they are flat. like this:

---------------------0 0-----------------------

This gives us two options for glass mounted antennas. You can make a single element, make it about 1/4 wavelength long, and connect that to the center of your radio antenna feedline (the coax going to the radio). Then connect the shield, which is the ground, to the car body.

The other option, is to make two elements, each 1/4 wavelength long, and connect to them in the middle. The center of the coax to one, the sheild to the other.

So, if you are using the defroster grid, attach to it and cut it to be an element of about the right length (2.2-2.5 feet should be about right). If you don't have a defroster grid, you can use cheap metal foil tape and stick it to the window. Size isn't really too important, a thin strip would work fine.

Since we are only talking about receiving, none of the dimensions are very critical. On the transmitter end, having a very small mismatch can cause a large amount of power to be used up as heat instead of transmitted, and since they are dealing with thousands of watts it is something to worry about. On our receiving end, the amount of power we are talking about is tiny.. a strong signal might impart a few microvolts to the input of your radio, which is why most of the radio is just high gain amplification.

One last note, Broadcast radio is almost always transmitted with vertical polarization. A vertical antenna will receive that signal with greater efficiency than a horizontal one, by about 6dB. Don't worry about the specifics, just know vertical is better for a car antenna.

So the short version? Stick some metal tape to one of your windows. If you attach the shield to the body, you can use 2.2-2.5 feet. If you don't, use two pieces, each 2.2 - 2.5 feet. Run the antenna vertically if possible. Connect to your new antenna using 75 ohm coax available just about anywhere (RG-59 is extremely cheap at home depot). I like copper tape, but I've used the metal tape sold for insulation purposes at home depot. The best way to connect to it is solder, but you could probably leave "tabs" to connect to with a twist-n-tape connection if you had to.

I've used the metal foil tape on my car windows to transmit 50 watts @ 144 Mhz before and had no issues, so it will work well for receiving. That same setup contacted the ISS Should cost you less than 5 dollars.

Your chances of damaging the radio are just about nil. The worst thing that could happen is that you short the antenna center to the ground, which would only result in crappy reception, and would easily be fixed by getting rid of the short.

Hope I didn't bore you all to death!

I should have skipped to the short version, my head hurts now :P

[jamesqf]

Don't know if they're made any more (I don't think I've listened to radio since CDs became affordable), but there used to be aftermarket antennas that were thin wire embedded in tape that you could stick on the inside of your windshield.

[Gregte]

Another thing to consider is to simply connect your existing defroster grid to your radio's antenna cable thru a capacitor. That way there is no need to remove its fuse or functionality.

Actually, the AM and FM receivers in your radio are probably both already capacitively coupled (internally) but putting a 0.1 microfard cap in series with the antenna coax's center conductor will ensure that the DC that powers the defroster cannot enter your radio. Connect the outer conductor (shield) of the coax to car body.

This does not guarantee the best radio reception possible but:
1) it may be as good as the existing antenna, which is a compromise by necessity.
2) it may be more than adequate for your needs if the signals you are tuning to are already plenty strong enough, which is most often the case.
3) a somewhat poorer antenna can be an asset if signal overloading is a problem, i.e. you are too near strong transmitters, overloading the receiver's front end.

[gteclass]

I have had great luck by just taking a peice of wire and stripping the ends and stuffing it into the antenna hole in the deck with the antenna wire and grounding it to the car. I have no idea if this is radio science correct, but it sure improves my reception in every car i have ever had even without an antenna mast.

[redline5th]

On most honda's that have an antenna like the thead poster, you should just be able to push the antenna down in the base. Still have the antenna and less drag. Just a suggestion I thought I'd throw out.

[RiseAbove]

Switch to XM or Sirius Satellite Radio. Great selection of tunes and no need for an antenna. The little satellite thingy is the size of a quarter and can be put anywhere.