Off-Center-Fed vhf/uhf vertical dipole

[BenB]

I just made one of these, and wow does it work well. Even mounting horizontally at the top of a window to hide it. I'm tempted to try it with aluminum tubing and put it out on my patio to see if it'll make more of a difference.

Living in an apartment my options are limited.

 

[basprog]

Ferrite choke on OCFD dipole

Nerd notes follow!

I placed a Radio Shack clamp-on cable choke (# 273-105) on the feedline near the feedpoint of my ocfd dipole.

Since the antenna is unbalanced, and the balun is a voltage balun, there is some interaction with the feedline braid. With the analyzer, I saw that placing this choke on the feedline brought the resonance points higher a few mhz - as it should.

This may not be important for general purpose reception, but since it did have a visible affect, so I decided to leave it on. I'm not sure if the RS choke is made of the right material for vhf, but it only seems to help.

This is not a big deal for most, but might be a nifty trick to try if you are monitoring a station just at the noise level to see if choking the braid might change the directional pattern a little bit, or bring the swr loss down just enough to allow for decent copy..

I haven't had this much FUN with an antenna in a long time ...

 

[FPOWLD]

Is it OK to use RG6 coax for the 48" and 18" elements or should I use some other wire?

Thanks

 

[TeRayCodA]

Use only single conductor wire,unless you use the shield of the coax,which IMO is not practical.

 

[Uplink]

Here's mine in the attic, I have about 40 feet of RG6 quad shield with F-connectors, a couple of cable clamps, an F-barrel, balun, lugs, and some old wire.

This has got to be the best scanner antenna for the buck. I hope I don't go broke making these.

 

[FPOWLD]

Well I did it. I made one of these antennas and mounted it horizonical in the attic. Seems to be working just fine. I see some mount it horizonically and others vertically. Anyone know which way is better?

Thanks

 

[BenB]

I did mine horizonally so I could span the top part of a window in order to hide it. I might tube it and mount it ouside on my patio, if I do that then I'll do it vertically.

 

[Bill2k]

If most of the transmissions you're listening to are transmitted from vertical antennas, like an antenna on a car, then mount it vertically.

 

[Uplink]

Two-Way radio transmissions are VERTICALLY polarized. (Unless you are only interested in receiving Broadcast TV, then mount it horizontally.)

 

[tonsoffun]

A little clarification:

Aluminum/copper tubing construction.You will need to "press fit" the tubing/"T" connector at the store for proper fit.The tubing/connector is held in place with 2 S/S sheet metal screws for connecting the balun.

Find a "U" bolt to fit your mast.Drill two holes in the support pipe to fit the U bolt.The support pipe is 18" from the "T" to the mast.

Remember,bandwidth increases as diameter of the elements increases.
I think,if I remember correctly,at the hardware store,that a few CPVC fittings will fit copper tubing perfectly!

Some say that the 18" element on top mounted works best,Some like the 48" element on top.It does'nt matter,it works the same.

If you use the copper tubing,be sure to paint it with some good,non-conductive paint.I used to paint mine light grey.
-Have fun!

Hey TeRayCodA
How do you make a solid connection for the Balun wires to the copper pipe, just with the SS screws?.
Looks interesting and going to make one for sure.
Thanks

 

[basprog]

Mini-version (110 mhz +) works well!

Thinking I could create a smaller version for those not interested in low-band vhf, I cut an ocfd for 120 mhz to see how well it works. Results - great all the way up to 900 mhz. (as well as a dipole can do however) I also ran some comparisons to the original ocfd and to a RS 20-176 groundplane.

Overall length of mini ocfd dipole: 48 inches
Short element length: 13 inches (nearly 27.5 % off-center from one end)
Long element length: 35 inches
4:1 balun, 300-75 ohm

Antenna analyzer shows that this one has the same properties of all the other ones I have tested - resonance at design frequency, a super resonant point 10 mhz below the fundamental resonant point, and swings up and down of no greater than 3:1 above resonance. Anything lower than 10mhz under the fundamental resonant point starts to go batty with worse case 5:1 and higher and lots of reactance.

Surprisingly, low-band vhf reception is still very good on this shorter version even though analyzer says that it should be pretty poor. I still have much listening to do, but casual operation suggests that these two ocfd's are very similar.

Compared to the original longer ocfd (5.5 feet overall length), the signal strengths were the same (except for maybe a bar-flicker difference here and there)

I also compared it to a Radio Shack groundplane 20-176 and results are the same, except on low-band vhf where this model groundplane is poorer by design on vhf-low.

So, if you find yourself constricted by the 5.5 foot length of the original and need it a bit shorter at 4 feet overall, this version works fine. I still need to do more listening and see if the directionality of the shorter version benefits my situation. I suspect lower angles, but we'll see ....

 

[TeRayCodA]

Hey TeRayCodA
How do you make a solid connection for the Balun wires to the copper pipe, just with the SS screws?.
Looks interesting and going to make one for sure.
Thanks

Yes,drill pilot holes,the balun makes continuity with the stainless steel screws through the insulator(pvc "T")

 

[FrankJ]

I just made one of these dipoles with both ends cut just over 9" for the center of the 225-400 Mhz Military Aircraft Band. Wow! It works great! I live in the Cleveland, Ohio area where there is generally not much Milair traffic. I'm hearing all kinds of stuff now.

BTW, I also use the Grove Omni-II. I'm very impressed with it's 25-1300 Mhz full range coverage. It does well in the VHF Low band also.....

 

[basprog]

Ah, FrankJ you made me get out my cutters again..

I just tried making a balanced dipole like you did with a 300-75 ohm balun at the center. Ordinarily I wouldn't have thought about using a balun and just connected it directly, but my 36" dipole with a 4:1 balun shows a 1.8:1 swr at 158 mhz resonance. The MFJ analyzer likes it! It also appears that placing rf chokes on the feedline has no effect, (unlike the ocfd) so that's a good thing. Thanks - I think I'll be making more balanced dipoles with baluns.

I'd be interested to see how an ocfd version of your dipole works by placing the balun 5.25 inches in on the short leg and about 13.5 inches for the long leg and comparing results. I just wish my instruments went beyond 170 mhz!

Thanks again for the homebrew inspiration..

 

[basprog]

Radio Shack Telescoping Dipole Kit

In the quest for the perfect ocfd dipole, I got tired of cutting wire. So I picked up a "back of set" telescoping dipole (#15-236) and hung it up vertically.

Each leg is 9 inches collapsed, and 40 inches each when extended. It comes with a 300:75 ohm balun and some twinlead. That's a bit flimsy for me so I just cut off the included twinlead at the feedpoint, and attached a more robust indoor/outdoor balun (#15-1140B) at the feedpoint instead. Unscrew the elements and slide the balun lugs underneath. It was a tight fit so I did away with the little copper pressure washers. Reassemble, and save the included plastic balun for a more dedicated install.

I taped the balun next to the plastic extension to provide some strain relief and hung the dipole with a tie wrap to a nail as I experiment with center-fed and off-center-fed dipoles. It works great and is a lot easier to experiment with now that I'm not cutting wire all day.

Have fun!

 

[Uplink]

Wow, Basprog! Very cool! Now you got me digging in my closet for old rabbit ears!!!

 

[FrankJ]

Basprog,

We must be on the same wavelength! (no pun intended) I just bought the VHF Dipole today on my way home from work! I also figured it would be easier than cutting wire. I was sold when I saw the collapsed length of the whip is around 9" (perfect for the 225-400 Milair band). I haven't hooked it up yet. It's nice to know I can adjust it for any frequency that I want. I love to experiment with antennas! I'm a freak!

Frank

 

[basprog]

FrankJ - It sure is a lot of fun. Here's some quickie starting points for ocfd's using 300:75 ohm baluns:

Typically they are fed 1/3 of the way in (0.333) of the total calculated half-wave length as opposed to the center-fed. They work well on *even* harmonics. (2nd, 4th, 8th harmonic - avoid the 6th however)

In order to use most of the antenna as an ocfd, pull one extension out to 40 inches, and pull the other out to only 20 inches. It's still not a great low-band antenna, but it sure beats ducks. Works ok higher up the spectrum but I favored the following two dimensions:

14 inch short leg, 28 inch long leg. Nice low swr at 125 and 135 mhz. Works great at higher freqs but all I have is an s-meter to go by.

I am also happy with 9 inches and 18 inch legs.

In your case for mil-air, I'd be interested in comparing your 18" dipole to an ocfd cut for your lowest freq of interest at 225. Maybe extend one leg to 16 3/4 and the other to 8 1/4 to compare how well it works up to 400 mhz..

Didn't mean to preach, but I'm so jazzed with mine I had to let it out!

 

[FrankJ]

I plan on using the Radio Shack VHF Dipole this weekend. I will compare to my UHF Milair home-made dipole. BTW, why is it called a "VHF Dipole"? Based on the length of the telescopic whips, shouldn't it work on UHF also?

Frank

 

[basprog]

Most people are using the whips for TV fully extended which is resonant in the vhf band.

It would work for UHF but not really well as when fully collapsed, the dipole would be resonant around 300 mhz which is near the low end of UHF. Great for mil-air but not so good for the UHF TV freqs which are much higher.

Good luck with your test. I'd be very interested in the comparison.