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-   -   Antenna identification question (http://www.risingsun4x4club.org/forum2/showthread.php?t=6320)

Mendocino 03-01-2008 09:44 AM

Antenna identification question
 
I have an old antenna that I used when I had a company radio in my truck years ago. I don't remember what part of the spectrum it used. The antenna length is ~59." Using the 468/λ=Length (in Ft), I then solved for what I knew: 468/4.958(antenna length in Ft)=94.38λ(in MHz). Then I divided 300/94.38 and got 3.14 (3 meter band).

So, if I did this correctly I don't think I can use this antenna for 10 meter of 70 centimeter with any reasonable level of efficiency. Please comment and feel free to correct any sloppy logic.


:cheers:

Mendocino 03-01-2008 09:53 AM

Hmmm..., looking into this further I could cut 12" off the thing to make it 47" in length and have a nice 5/8 wave 144MHz antenna:)

Seldom Seen 03-01-2008 04:54 PM

Does it have a loading coil ? If so, all bets are off when it comes to the math.

Could be a 5/8's on VHF high or 1/4 on VHF low business bands. Best way to tell is, hook it up to a MFJ antenna analyzer and see where it's resonate.

Either way, I'll bet it could be cut to 2mtrs.

Mendocino 03-02-2008 10:26 AM

It has a stainless steel spring at the bottom of the antenna, but I am pretty sure this is for mechanical flexibility of the antenna (shock spring). The antenna element does not have an "in line" coil or trap.

Shorty 03-03-2008 01:55 PM

Quote:

Originally Posted by Mendocino (Post 64435)
I have an old antenna that I used when I had a company radio in my truck years ago. I don't remember what part of the spectrum it used. The antenna length is ~59." Using the 468/λ=Length (in Ft), I then solved for what I knew: 468/4.958(antenna length in Ft)=94.38λ(in MHz). Then I divided 300/94.38 and got 3.14 (3 meter band).

So, if I did this correctly I don't think I can use this antenna for 10 meter of 70 centimeter with any reasonable level of efficiency. Please comment and feel free to correct any sloppy logic.


:cheers:

Please correct me if I am wrong, but isn't the equation that you gave for finding the length of a dipole? That is, with the 468 factor in it. For a 1/4 wavelength (how do you put a lambda in here anyway?) plane-wave antenna, I believe that the factor should be 234. This would give a frequency of 47.2 MHz and a wavelength of 6.36 m; hence, the antenna is for the 6 m band.

DaveInDenver 03-03-2008 02:48 PM

The rule of thumb to find a quarter wavelength monopole antenna is using 234/frequency (in MHz). For 1/2 use 468/f and for 5/8 use 585/f. So a 1/4λ antenna on 2m is 234/145 = 1.61 feet and a 1/2λ is 468/145 = 3.22 feet. If you are making a dipole, then each half would be 1.61 feet and the overall length would be 3.22 feet, if you are making a 1/2λ vertical then the whip would be 3.22 feet. Keep in mind that only the 1/4λ monopole and 1/2λ dipole have the correct intrinsic feedpoint impedance to match a coax, the 1/2- and 5/8-wavelength antennas have much higher feedpoint impedances and will need to have a matching network to work. So if the antenna in your hand does not have a matching network, auto-transformer or something to match the coax impedance then the antenna can only work as a 1/4λ whip.


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