Home made antenna for VHF

[Prospect62]

Guys, I stumbled across the thread about the "home brew" OCFD antenna and after reading through a ton of posts on it, I'm wondering if a home made dipole of some kind might be better for me?

I'm monitoring VHF high band ONLY 154-159 MHz region.

If so, could I use these basic instructions...

https://wiki.radioreference.com/index.php/Homebrewed_Off-Center_Fed_Dipole

But with equal length sections on each end? I'm not sure about theory and resonance and all that stuff. Just looking to experiment with the home brew antenna and since I'm only listening to a limited section of one single band, I wondered if a true dipole would be more effective than an offset.

Thanks for any input.

 

[riccom]

i would go with a 18" whip with 4 20 inch ground radials, it will be centered at 155.880 and for just monitoring, it will work like a champ,

 

[nanZor]

Yes, a simple dipole would work better for you in this single-band application with each side the same length, and NO balun necessary.

Find your half wavelength in feet: 468 /f mhz (468 / 156.5) = 2.99 feet total. Keep it easy at 3 feet total.

Cut this in half so that you have 1.5 feet each side. Attach braid to one side, and center connector to the other. Try to keep the leads attaching to the wires / tubes short.

Run the coax horizontally away from the vertical dipole for at least 3 feet, and then bring it down however you need. This helps keep the coax from coupling to the dipole, and wrecking the pattern.

It's a classic simple antenna that I'm sure you'll enjoy.

 

[ko6jw_2]

This design has been the subject of another thread quite recently. The question you have to ask is why you would build this antenna as opposed to a j-pole? Vertical dipoles, off center or not, are not as simple as they appear. Contrary to other comments, a dipole, regardless of the feed point, is a balanced load and should not be fed directly with an unbalanced line. The distance between the vertical dipole and the support will affect the radiation pattern. The off center feed will have a higher impedance than the center feed. This may explain the use of the 4:1 balun in the design in question. However, there is little or no difference in performance all else being equal. Since you are interested in a specific band, a j-pole would be an easier project without the complications of the vertical dipole. You might consider the ground plane as well. Even easier to build and the performance difference will probably not be critical.

There are commercially available vertical dipoles made by Telewave among others. We just installed a two bay vertical dipole array at our repeater. The directional pattern is set by the spacing between the dipoles and the tower. The resonant frequency is a function of the phasing harness connecting the two dipoles. About 6dbd gain. A little over $1,000. Not a DIY project. I bring it up only because I don't want to suggest that a vertical dipole is a bad idea, just not an easy one.

 

[Prospect62]

Attach braid to one side, and center connector to the other. Try to keep the leads attaching to the wires / tubes short.

I don't know what you mean by this...

**EDIT never mind I got ya. Is solder the way you connect the braid and center wire to the poles? And I assume we stick with 1/2" copper pipe here?

Can you explain why the balun wouldn't be necessary?

 

[Prospect62]

I've read some things which indicate that even a balanced dipole needs a balun as the coax is unbalanced. Would it hurt anything if I used a basic 300/75 TV balun to connect my coax to the dipole?

 

[ChrisABQ]

I have been using my homemade 1/2 wave dipole for two years now. Made from aluminum, I have 2 equal lengths of 3/8" tube separated by about one inch. Each tube length is about 19" for my particular range in VHF. I also am able to receive about 20 mhz above and below my target frequency. Living in Albuquerque, I can receive up to 80 miles South of the city and up to 100 miles out Northwest. Has functioned far better than anything that I have bought (HOA restrictions exist) and has been rock solid.

 

[Prospect62]

I have been using my homemade 1/2 wave dipole for two years now. Made from aluminum, I have 2 equal lengths of 3/8" tube separated by about one inch. Each tube length is about 19" for my particular range in VHF. I also am able to receive about 20 mhz above and below my target frequency. Living in Albuquerque, I can receive up to 80 miles South of the city and up to 100 miles out Northwest. Has functioned far better than anything that I have bought (HOA restrictions exist) and has been rock solid.

Are you using a balun of any kind or did you just solder the braid to one side and the center connector to the other?

 

[nanZor]

I've read some things which indicate that even a balanced dipole needs a balun as the coax is unbalanced. Would it hurt anything if I used a basic 300/75 TV balun to connect my coax to the dipole?

No, BUT now you have an impedance mismatch - the antenna is 50-75 ohms impedance, but the 300/75 ohm transformer is a 4:1 ratio, making the coax want to see a 300 ohm antenna! Which the simple dipole isn't.

Let's keep it simple and not go nutzo with perfectionist details for this first-effort application. You can get into that later and improve as you go. Or you can be swayed by the best of intentions and *never* put anything up because others can over-engineer things to perfection. There's a place for that, but not right now.

Just attach your braid to one side (screw into pipe, what have you) and center conductor to the other. Run feedline horizontally away from it to minimize the braid coupling. The reason for keeping the connections short is that if they are long, say 6 inches when splayed out, they are part of the antenna, and now it is longer than you originally calculated, and not resonant at your desired target frequency.

This may be all you need even if it isn't absolutely perfect from an antenna-geek standpoint. Try it. Have fun.

Want to improve it? Instead of using a mismatched transformer, the problem of a balanced antenna to an unbalanced coax can be cured by choking it with ferrites near the feedpoint on the coax.

Grab some #43 snap-on ferrites (about 4 or more), with a size large enough to clamp over the coax, and put them near the feedpoint. This will reduce the coupling even further - which may not even be necessary for your first-effort scanning needs on VHF.

What #43 means is that the "mix" of the ferrite material works best from about 30-300mhz, which is what you'd want for your 156mhz or so dipole.

But don't let that stop you from simply trying it first. That's part of the learning fun.

 

[Prospect62]

Thanks Hertzian. Great explanation. I’ll post my results.

This dipole is going in my attic by the way. We shall see how it does!

 

[Prospect62]

Well, here’s what the ferrite company says...

“For RFI use, mix 31 is effective from 1-300 MHz, mix 43 works from 25-300 MHz, Mix 61 is for 200-2000 MHz, and mix 75 favors .15-10 MHz. These frequencies are those of the interfering signal to be eliminated, not the operating frequencies of the equipment to be protected.”

So that doesn’t make sense - going by that statement, these ferrites would filter out the frequencies I actually want. Or am I reading that wrong?

 

[nd5y]

If you are using them for a choke or balun then
"These frequencies are those of the interfering signal to be eliminated, not the operating frequencies of the equipment to be protected.”
are the same. You want it to filter common mode currents on the outside of the coax at your operating frequency.

 

[nanZor]

Yep - ND5Y is right. The ferrite chokes go on the coax, close to the antenna.

This way, if the coax braid being excited by not having the right connection (balanced antenna to unbalanced coax), the ferrites attenuate the problem caused by that on the outside of the feedline but they will NOT attenuate you antenna performance! So you are good, unless you did something wrong, like slipping a choke over one of the dipole tubes!

And as such, the braid doesn't couple to your antenna, skewing your directional pattern. If you have the ferrite specs, one typically wants at least 2K, 2000 ohms or more of resistive "choking impedance", so you need about 4 or more stacked right behind each other.

But to be sure, if you don't have the ferrites on hand, or don't feel like waiting for them, don't hesitate to just put the antenna up anyway, and take a listen.

 

[Prospect62]

But to be sure, if you don't have the ferrites on hand, or don't feel like waiting for them, don't hesitate to just put the antenna up anyway, and take a listen.

Makes sense, and no I don't have them on hand nor do I want to spend money on them. I'll throw the dipole I made up and see how it does.

Thanks you guys for all of the great advice!

 

[Prospect62]

So I finished my VHF dipole made out of 3/4" copper pipe, 18" on each side. Skipped the ferrites for now.

I kept the 300/75 TV balun on there, put the antenna up in the attic oriented vertically with 40' of RG6 (with about a 1.5 feet of it leading horizontally away from the antenna, I didn't have an easy way to go 3 feet). It's been working amazingly well.

I could probably be pulling more in if it was outside, and who knows maybe the balun is holding it back? I'm satisfied for now. Works great.

Thanks to all. If nothing else I learned a few things.

 

[riccom]

Good deal, and you can add on or make minor changes over time or keep it as is, welcome to diy!

Sent from my 5049Z using Tapatalk

 

[15plus1]

So I finished my VHF dipole made out of 3/4" copper pipe, 18" on each side. Skipped the ferrites for now.

I kept the 300/75 TV balun on there, put the antenna up in the attic oriented vertically with 40' of RG6 (with about a 1.5 feet of it leading horizontally away from the antenna, I didn't have an easy way to go 3 feet). It's been working amazingly well.

I could probably be pulling more in if it was outside, and who knows maybe the balun is holding it back? I'm satisfied for now. Works great.

Thanks to all. If nothing else I learned a few things.

Sounds neat. Post some pics if u can...

 

[prcguy]

I would shorten the antenna to 16" due to the fat elements pulling the resonant frequency lower than what you would calculate. If you look up my "low cost high performance VHF dipole array" project on RR you will see I came up with 16 1/4" long elements made of 3/4" copper tubing and my dipoles were for a hand tuned slightly lower frequency than you are targeting.

You also need to loose that TV transformer as it is absolutely having a negative effect on your reception. Make a direct connection of the RG-6 right to the elements with the shortest pigtail possible. Later you can get some #43 mix snap on ferrite cores and place three of those right at the feedpoint.

 

[Prospect62]

I would shorten the antenna to 16" due to the fat elements pulling the resonant frequency lower than what you would calculate. If you look up my "low cost high performance VHF dipole array" project on RR you will see I came up with 16 1/4" long elements made of 3/4" copper tubing and my dipoles were for a hand tuned slightly lower frequency than you are targeting.

You also need to loose that TV transformer as it is absolutely having a negative effect on your reception. Make a direct connection of the RG-6 right to the elements with the shortest pigtail possible. Later you can get some #43 mix snap on ferrite cores and place three of those right at the feedpoint.

Interesting. Most people have said the balun won't help in this case but it won't hurt either. You're the first person to say it would actually worsen my results. Worth a shot at trying it without the balun I guess.

Perhaps someday...as of right now it hears what I need it to hear very well. Any improvement would be neat, but not necessary. Thanks again everyone, I'll post my results if I change anything.

 

[prcguy]

A TV transformer converts 75 ohm coax to 300 ohms but the dipole you made is around 70 ohm impedance. That mismatch will produce loss and the transformer also has some inherent loss even with a 300 ohm load. I would estimate you are loosing at least 2dB of signal with the TV transformer in line.

Interesting. Most people have said the balun won't help in this case but it won't hurt either. You're the first person to say it would actually worsen my results. Worth a shot at trying it without the balun I guess.

Perhaps someday...as of right now it hears what I need it to hear very well. Any improvement would be neat, but not necessary. Thanks again everyone, I'll post my results if I change anything.