Question about dipole antenna

[cjrjr507]

I want to possibly build either an inverted v dipole or regular dipole antenna that would cover from 25 mhz - 88 mhz for listening to skip transmissions. If I cut the dipole to a certain center frequency, how much on either side of the center frequency in mhz would the antenna be able to pick up. Say I cut wires for 25 mhz, 35 mhz, 45 mhz, 55 mhz, 65 mhz, 75 mhz and 85 mhz would the wires cover 10 mhz on either side of the center frequency? I know that 25 mhz is 9.36 feet for 1 leg of the dipole or 18.72 ft for both legs, would that cover the entire band ? Also would I need a balun at the feed point ? I am just receiving with my BCT15 scanner. If I do need a balun I have some 300 to 75 balun lying around not being used. Thanks for any info.
I have been doing research on making one.

 

[kilowa22]

Just cut for center frequency of where you want to listen. You don't need a balun at all since you will not be transmitting. You could cut for 40 MHz and that will give you enough bandwidth for monitoring the low bands. I have used dipoles cut for 28 MHz to monitor 30-50 MHz on a Ham radio with great results.

 

[jackj]

Some antenna info

I want to possibly build either an inverted v dipole or regular dipole antenna that would cover from 25 mhz - 88 mhz for listening to skip transmissions. If I cut the dipole to a certain center frequency, how much on either side of the center frequency in mhz would the antenna be able to pick up. Say I cut wires for 25 mhz, 35 mhz, 45 mhz, 55 mhz, 65 mhz, 75 mhz and 85 mhz would the wires cover 10 mhz on either side of the center frequency? I know that 25 mhz is 9.36 feet for 1 leg of the dipole or 18.72 ft for both legs, would that cover the entire band ? Also would I need a balun at the feed point ? I am just receiving with my BCT15 scanner. If I do need a balun I have some 300 to 75 balun lying around not being used. Thanks for any info.
I have been doing research on making one.

One thing you need to remember, if it does it on transmit it will do it on receive. For the best performance, you need an antenna that is matched to your receive frequency. Yes you will be able to receive 88 mhz on an antenna cut for 25 mhz but not as well as one cut for 88 mhz. The antenna's radiation lobes will be hard to predict over that great of a frequency spread.

Try cutting several lengths of wire for different parts of the spectrum you want to monitor. Connect them all to the same center feed point and use some type of spreader to keep them separated. That will give you several different antennas connected to one feed line.

The balum you have is probably for TV and is a 4 to 1 balum. You need a 1 to 1 balum for this antenna if you decide to use it. One isn't absolutely necessary but it won't hurt anything either and will eliminate any currents on the coax shield that would tend to distort the radiation pattern.

 

[LtDoc]

A very general 'ROT' (rule of thumb) is that longer antennas tend to do better than shorter antennas. Several antennas, one for each band of interest, is a good idea too. If you combine all of those antennas so that they have one feed point it certainly makes things easier.
Receivers are just not as 'picky' about things as transmitters are. Sometimes you can 'over-build' without much trouble, but that 'over-building' isn't always very beneficial. I figure baluns are sort of a 'stretch' for receive only antennas. ("currents on the coax shield that would tend to distort the radiation pattern"... what currents?) If you want to try a balun, do so! Then try it without that balun and tell us the difference. The biggest improvement is typically seen by getting that antenna higher.
- 'Doc

 

[prcguy]

A dipole cut for 40MHz or any frequency within your band of interest will work fine within a few MHz of its design freq, then in will degrade as you go further away until its really bad at some point.

The range your looking for is essentially what the military uses for VHF lo-band tactical communications and its also something I've been looking into for many years to find the perfect antenna.

My version of the perfect VHF lo-band antenna is the military OE-254 bicone, since I can't afford the newer COM-21B that replaces it. You can find the center hub for the OE-254 on Ebay anywhere from $15 to $75 and instead of the bag of screw together elements you can install permanent aluminum elements from the hardware store.

The original 254 is rated for 30 to 90MHz using about 8ft elements and with 10ft elements it easily covers from 10M/CB to probably 75MHz continuous. Mine replaced a separate 5/8 wave 10m and 1/2 wave 6m antenna and its working about the same. VHF lo-band reception is really hot compared to any other antenna I've used.

The center hub contains a modified 4:1 balun and holds the elements at the critical angle to make everything work. Here is a link to a pricey center hub and another with an insulating fiberglass mast section which is needed:

Radio Discone Element Good Used Military Radio - eBay (item 220716332132 end time Jan-27-11 12:45:20 PST)

Antenna Head&Fiberglass Insulator Radio Part - eBay (item 360313030777 end time Jan-21-11 11:57:11 PST)

Here is a link to my OE-254, it should be post #10 in the thread:

http://forums.radioreference.com/antennas-coax-forum/134540-my-homebrew-bicone-antenna-project.html

Here is a link to a discussion on the OE-254, making a field expedient version and comparing it to its replacement:

Radio communications, light infantry and MOUT
prcguy

 

[W2NJS]

Forget about the 300 ohm/75 ohm balun. If you use single wire the impedance of the feedpoint is 75 ohms; if you use 300 ohm twinlead for the legs the impedance is then 300 ohms and then you need the balun to go from 300 to 75 ohms. I would cut the antenna for the lowest frequency you intend to use, but if you have the space you can actually use multiple dipoles of different lengths connected to the same feedpoint in a "fan" arrangement. This is done all the time with multiband antennas.

 

[cjrjr507]

Thanks for the info. I won't be able to build it until the ground thaws in spring. Heh heh.

 

[majoco]

Read all about OCFD's in the RR Wiki then roll-yer-own!

 

[n5ims]

What I have done in the past is get some flat multi-conductor rotor wire and cut them to various lengths in the range you're interested in listening to to provide somewhat a multi-band dipole. Since you won't be transmitting, the lengths shouldn't be too critical, just space them somewhat evenly throughout the desired frequency range to make it more of a wide-band antenna.

Here's a write up on something similar for you to base your design on 6 Band Wire Antenna

 

[rfradioconsult]

Remember you want to install your dipole vertically, not as a conventional horizontal dipole. Public Safety, Ham and most other services use vertical polarization.