Antenna length calulation not corresponding to my 7 element telescopic?

[rockblock]

Hi Folks,

I'm new to antennas. I have a GRE PSR800 scanner. I just received a generic telescopic, 7 element, 6.5-24.5" (including BNC) elbow join antenna. According to technical data, this is supposed to cover between
20-1800MHz.

As I was trying to figure out what elements/lengths to use for which frequency ranges, I ran into this site which provides an Antenna Length Calculator.

Based on the calculator, the optimal length for say, 862MHz would be 3.25". Now the lowest I can fold my antenna to is 6.5", so I'm aware that I can not get the optimal reception. But when I extend the antenna to it's full length, I actually noticed that I'm able to gain an additional signal bar with less static for the 800MHz range...

So the results I obtained are inverse-proportional to the calculator data, hence my dilemma... do telescopic antennas follow a different method of calculation? How would I know what element extension to use for each frequency range for this antenna?

Thx in advance

 

[WA1ATA]

The calculator you used calculates 1/4 wavelengths.

Antennas will work well at other wavelengths, such as 5/8, albeit with different patterns.

Radiaton patterns for antennas longer than about 5/8 wavelength have multiple lobes and rather irregular patterns. If you are just trying to receive one particular station you can sometimes adjust the antenna to take advantage of this.

If you can adjust the antenna to the 1/4 wavelength length that is best because it has a nice repeatable pattern that doesn't have the deep notches in reception found with very long wavelength antennas.

 

[K9WG]

I am sure that the supplied antenna is a stock telescoping one and not specifically engineered for the scanner.

 

[majoco]

Random lengths give random results: On a handheld, the "ground" (ie the radio and it's surroundings) has a great effect too on signals strength received. Try standing your radio on a metal plate and you will get different (maybe better/maybe worse!) results.

 

[WA1ATA]

I am sure that the supplied antenna is a stock telescoping one and not specifically engineered for the scanner.

And the relevance of this is ..... ?

 

[rockblock]

Well, I didn't expect much from a $10 antenna, but I was intrigued to see it perform opposite to what the theory said...

On the other hand, I just cut a 4" coax cable and stripped 3.25" off it and it works the same as my telescopic one...looks like the formula applies to a simple copper wire, but not to my telescopic

I know I'll have to go with a tuned telescopic (i.e.Watson W889, Diamond RH795) - was just curios as to why this generic telescopic would perform in contradiction to the theory (longer for lower, shorter for higher)

Thx for all you feedback !

 

[LtDoc]

Very basic reason is that receivers generally don't rely on the antenna's impedance (major reason why specific lengths are used with transmitters). To some absolutely ridiculous extent, longer is better. If antenna length is harmonically related to frequency wave length, they tend to work 'better' than non-harmonically related antennas.
There's a lot more to it, but that's a fairly accurate general reason.
- 'Doc

 

[prcguy]

A 3/4 wavelength antenna is close in impedance to a 1/4 wave and many VHF/UHF dual band antennas rely on this since their simply a 1/4 wave on VHF.

The 3/4 wavelength radiation pattern has a slight null at the horizon and there is a little gain in the upward pointing lobes, but it generally works fine.

So, if a 1/4 wave whip is about 3.25" long at 800MHz then 3/4 wavelength at about 9.75" would work fine also.
prcguy

 

[W2PMX]

Actually, any odd number of quarter wavelengths will look like something around 50 ohms. That's both theory and practice. But a scanner antenna working against nothing (no ground plane) is a random length even if it's a quarter wave, so the theory of using a quarter wave vertical no longer applies. Especially since most scanners have the antenna jack somewhere on the back panel, quite a distance (for 800 MHz) below the ground plane the cabinet creates (but to one side only). The rule (theoretical and practical) that the larger the capture area the more signal received applies here, so the longer the antenna the better for most frequencies. (Horizontal pattern - which is about impossible to calculate for an antenna like that) also has a lot to do with what you receive.

Whether the antenna is a 3.25" length of coax center conductor, a 3.25" long telescopic antenna or a 3.25" long wet string, the theory is the same. (Within limits - the prop speed of a wet string is probably not the same as the prop speed of copper.)

 

[rockblock]

Ok, I think I now understand...but won't a bigger antenna also attract the extra, unwanted noise, static, etc? I guess the trick would then lie in how to filter out that noise, hence tune accordingly?

Interestingly, by placing my scanner on a large tin can (1liter/33oz), with the telescopic fully extended, I can increase my 800MHz signal strength by 1 line (in certain areas of the house) - I would also get additional channels that I could not previously receive, unfortunately they come in with so much noise and distortion that it locks the scanning on that channel

 

[prcguy]

Scanners, handheld transceivers and most any radio provides some sort of counterpoise for a whip that is directly connected to the radio. Your rand around the handheld also contributes some to the handheld antenna system.

When I worked for a very large but un-named radio company we had a few handheld frames with circuit boards and and feedline attached to the radio antenna connector so antenna measurements could be taken. The feedline was decoupled from the handheld radio frame with Ferrite chokes so the feedline could be ignored.

Various rubber duckie antennas we tested on the handheld frame resonated very close to where they were tuned on a larger ground plane.
prcguy

Actually, any odd number of quarter wavelengths will look like something around 50 ohms. That's both theory and practice. But a scanner antenna working against nothing (no ground plane) is a random length even if it's a quarter wave, so the theory of using a quarter wave vertical no longer applies. Especially since most scanners have the antenna jack somewhere on the back panel, quite a distance (for 800 MHz) below the ground plane the cabinet creates (but to one side only). The rule (theoretical and practical) that the larger the capture area the more signal received applies here, so the longer the antenna the better for most frequencies. (Horizontal pattern - which is about impossible to calculate for an antenna like that) also has a lot to do with what you receive.

Whether the antenna is a 3.25" length of coax center conductor, a 3.25" long telescopic antenna or a 3.25" long wet string, the theory is the same. (Within limits - the prop speed of a wet string is probably not the same as the prop speed of copper.)

 

[W2PMX]

Ok, I think I now understand...but won't a bigger antenna also attract the extra, unwanted noise, static, etc?

Yes, a non--directional antenna can't increase the signal to noise ratio. Antenna gain increases signal and noise by the same amount.

I guess the trick would then lie in how to filter out that noise, hence tune accordingly?

That's done in the radio, not in the antenna or feedline. Since the radio is FM in this case, as long as the signal is <the capture ratio> above the noise, you won't hear any noise. Capture ratio varies by radio, but it can be well under 2db (IOW, if the signal is as little as 2db above the noise, you don't hear the noise).