Dipole Freq.?

[daman1]

Plan on making one soon,,


First question what home made antenna would work best for Freq ranges 40-800 but more realistically <100-500+ range?

second question can the home made Dipole lengths be modified from the standard 18" and 48" to receive the desired freq? like shorten one element to receive lower band transmission ?

 

[jaspence]

Dipole antenna

A dipole is typically mounted for horizontal use. If your transmitting station(s) use vertical polarization, you are reducing your signal before it reaches the radio. Each leg of a dipole is usually the same length, 1/4 of the desired frequency. They are generally not designed for multi-band receiving use. When used for transmitting on more than one band, there are "traps" to allow transmitting on more than one band. A dipole has some advantages for ham radio where signals travel long distances due to reflection off the atmosphere but may be less desirable for local listening. See Dipole antenna - Wikipedia, the free encyclopedia

 

[popnokick]

The OP is referring to the Off Center Fed Dipole (OCFD) that is in the RR Antenna Wiki.
For the broadest frequency coverage it should be built as shown and mounted vertically to match most scanner comm polarities.
Shortening either leg will raise (not lower) the freq coverage in a usually undesired fashion. The OCFD scanner antenna has been documented extensively here on RR. Use the Search function to answer your questions about it... And answer many more.

 

[daman1]

I'm sorry yes this would be for receiving only, police scanner. Yes I have searched but with over whelming results.

ok so leave well enough alone.

 

[teufler]

Several have used an old tv abtenna for monitoring. A tv antenna that has ch 2 thru 13 plus a uhf section c anb cover closely most scanner frequencies better than an inside antenna. Even if is horizontal, it still does pretty good. Its one of those antennas that can be used in a pinch. Biggest thing will be getting th4e adapter to match your scanner.

 

[majoco]

Several have used an old tv antenna for monitoring. A tv antenna that has ch 2 thru 13 plus a uhf section can cover closely most scanner frequencies better than an inside antenna.

+1

except I cut the low band antenna down for the aircraft band.

 

[N3JI]

A single dipole for those frequencies won't do it. You could to a "fan" dipole though with elements cut for the bands you're interested in. But keep in mind that a 1/2 wave dipole will also be a good antenna at 3x the fundamental frequency. So for example, a 100 MHz dipole (with 1/4 wave legs) will work well at 300 MHz (3/4 wave legs).

 

[nanZor]

Plan on making one soon,,

The canonical dipole is merely:

468 / F in mhz = total feet. Now you can feed that length in the center, or anywhere else offset along it's length, but if you do, you'll want to match impedances to the coax with a transformer. It will work at the design frequency, and also 3x that, with a slight change in the directional pattern.

second question can the home made Dipole lengths be modified from the standard 18" and 48" to receive the desired freq? like shorten one element to receive lower band transmission ?

You are referring to the "OCFD" vertical scanner dipole. It has a natural resonance down near 88mhz, and some other points higher up in the bands. However, it is purposely designed to be "non resonant" on other bands. Resonance isn't always needed for efficiency, and being non-resonant on purpose, the 300:75 ohm (or 4:1 ratio) balun transformer helps bring the impedance down closer to that of 50/75 ohm coax on a multitude of bands.

The major problem is that above the VHF band, where the elements are significantly longer than an ordinary dipole, the radiation angle starts to head straight upwards. Not usually what one wants. HOWEVER, if you live in a very RF dense environment, powerhouse uhf and 800mhz signals will be ok. This pattern-attenuation if you will, also helps with the typical weak front-end of scanners prone to overload. Normally this antenna is not mounted in the most ideal places, and uhf and 800mhz reception can rely on strange angled reflections to help.

So in the end, it is an easy-up Saturday empirical goof project. It works for many as a first time bedroom antenna, far better than the usual back of set whip - but to be sure, you'll never see one on a hilltop connected to commercial gear.

Caveat - since this antenna is broadband, with a scanner you may also be susceptible to FM broadcast overload, or a multitude of other powerhouse signals that can desense the front end.

Essentially, you just try it, and if it doesn't work - ball it up and toss it.