Best Short Radio Antenna?

I got one of these last October at Walmart as the OEM antenna would not fit under our flat roof carport; it comes with adapter studs that fit the OEM base, I tossed the supplied base and cable into one of my junk drawers--who knows what you might need some day.

It is 13" long--1/9th λ at 100 MHz (the middle of the FM band) but while not an optimal length (29.5" is 1/4 λ) it does work well here i the flatlands of NE Florida. I get good reception of stations from Jacksonville and Daytona. both 50+ miles away; even when its bent backwards at 85 mph.

13" is about as short as you can go and have any reasonable FM detection and anything other that close-in suburban locations.

For $8.97 you cannot go too far wrong...

 

I replaced mine with an Antenna X that's 19 or 21"s long, can't remember. It's very solid, and gets the same or better reception as stock.

 

Generally, your antenna comes at the stock length because an engineer has figured out that is the optimum length to receive the AM-FM spectrum. If they could do the same with that antenna being shorter they would save money, so they would install a shorter antenna. They could get away a shorter antenna if they used an antenna with a different construction. I'm going to guess they don't due to cost.

So, if you are cutting your stock antenna without paying attention to the science then it is very doubtful you aren't negatively affecting it's ability to receive the full signal spectrum. How much? You'll need to do some math or use test equipment to see how much.

 

AM reception is pretty much a factor of antenna length--longer is better--always.

Here's the math for FM:

For FM antenna length is a significant factor (for reasons that would not really fit in this context, except to say that some even multiple or divisor of the target frequency wavelength is always best (x2, x4, 1/2, 1/4, 1/8, etc.)

In the US the FM broadcast FM band runs from 88 MHz to 108 MHz, placing 98 MHz smack dab in the middle.

Radio waves travel at the speed of light which is 299,792,458 m/s.

Therefore the "length" of one wave of a 100 MHz signal = 299,792,458 / 98,000,000 = 3.0591 meters = 120.4373 inches.

"λ" (lowercase lambda) is the symbol used for wavelength.

I.e. 120.4373" would be the length of a 1.0 λ antenna optimised for 98 MHz (center of the FM band) reception--best, but impractical for a vehicle\--as would be 1/2 wave, 60.2365".

1/4 λ is 30.1093". which not-coincidentally is the length of most automotive FM "quarter-wave" antennas (though some are bit longer, 31" is common, to favour the lower end of the FM band which for a variety of reasons does not "carry" as well).

The next "best" length would be 1/8 wave = 15.0547"--call it 15" for sale of argument--and in the real world it turns out that 1/9 wave (13.3819") works out nearly as well and shaves another 2" off the antenna length.

If you want to favour the high or low end of the FM band use 93 or 103 MHz (126.9125" or 114.5909") as the base λ.

Or you can pick your favourite station (say 103.9) and divide 299,792,458 by that (ex. 299,792,458 / 103,900,000 = 2.8854 meter) then multiply that times 39.3701 to get inches (2.8854 * 39.3701 = 113.5983"). For 103.9 MHz 1/4 wave antenna would be 28.3996", 1/8 wave 14.1998", 1/9 wave 12.6220"

Piece of cake..

 

I have one of these and no complaints. Antennax