Off-Center-Fed vhf/uhf vertical dipole

[basprog]

Put up a new antenna today and it is working very well for me.

I searched the web and read about attaching 48 inches of wire to one side of a 300-75 ohm balun, and 18 inches to the other side. I used a small jumper to go from the balun to the scanner. Wow - this thing works much better than my "coaxial dipoles" as a broadband dipole.

Time to read up on the OCFD's I guess.... Meanwhile life is good!

[nd5y]

You just invented the Grove Omni-II VHF-UHF Receiving Antenna
http://www.grove-ent.com/ANT5.html
and saved yourself $30

[basprog]

Wow. I wonder if anyone here is using one? My homebrew version seems to be doing really well, although my dimensions are different than his by about two inches. I'm actually thinking of doing a multiple ocfd dipole version to cover any gaps but that's for another day...

I put an MFJ antenna analyzer on it, and just wish I had more accurate instruments that went up to 900 mhz or so...

Setup: 66-inch vertical ocfd dipole up only 15 feet, with a 12-foot RG-58 jumper. F to bnc adapter inline with RS 300:75 ohm balun. Here's a quick selection of frequencies. Fascinating.

Freq     R     X     SWR
------  ---    ---   -------
35       9     12    4.9
40       21    24    3.1
55       21    37    3.6
60.6     27    4     1.8
88.1     20    1     2.4
112.8    16    0     3.0
120.0    80    56    2.8
135.0    16    0     3.0
145.0    67    51    2.3
152.7    21    0     2.3
160.0    103   0     2.0

This is where the 259 starts to run out of steam. At least the antenna doesn't!

[basprog]

Not being content to leave well enough alone, I thought I'd scale two more OCFD antennas for comparison, one being half the size of the original, and the other being twice the size at 11 feet. I used the same feedline and placement for the antenna.

The smaller version (24 inch long element, 9-inch short) worked ok from 112 on up, however I lost my weak-signal 800mhz reception on a known weak station.

The large version (8 foot long element, 3-foot short element) worked great. Improved my low-band reception near 42 mhz, but signals at 160mhz down a bit. What was surprising was that my weak signal 800mhz reception was just slightly better than my reference 800 mhz RS duck. That is, 800 works fine generally I was using a known weak station as a reference.

Both of these new antennas showed nearly the same properties as seen on the MFJ analyzer - at least up to 170 mhz which is the limit for my MFJ. (Mostly resistive resonance at design freq, two nice impedance bumps spaced 20 and 40 mhz below! and a few more mostly resistive impedances higher than design.) For me anything lower than 4:1 and mostly resistive is my goal...

It seems odd to build an OCFD for 88 mhz and have it turn out to be a nice general purpose scanner antenna, but for now I'm smiling!

I know there are a lot of other variables to consider, and If I find a nice combination for the 11-footer (to get 160 mhz back to par) I'll let you know....

[TeRayCodA]

Also,keep in mind that when the diameter of the OCFD's increases,so does bandwidth.

I once built one of these out of cans,soldered together(48",18") just for something to do.Worked great.

And,you can imagine using galvanized vent-pipe with some type of insulator.

For 800Mhz,I added a 3" stub on the feedpoint(as a parasite element) on one before.

[basprog]

This trial run is proving successful, so I may look into tubing - like you say it will help broaden the bandwidth of the various peaks.

Thanks for the parasitical element tip! That might help me from going crazy trying to encompass so many bands and finding only one band that needs some help.

BTW, I love those pics of your homebrew antennas. Wow! Must be a pleasure to use them.

[Uplink]

Originally Posted by basprog

Put up a new antenna today and it is working very well for me.

I searched the web and read about attaching 48 inches of wire to one side of a 300-75 ohm balun, and 18 inches to the other side. I used a small jumper to go from the balun to the scanner. Wow - this thing works much better than my "coaxial dipoles" as a broadband dipole.

Time to read up on the OCFD's I guess.... Meanwhile life is good!

Great antenna Basprog! Thanks! This $5 gatget outperformed my $100 Diamond dipole on Lo-VHF. A local 6-Meter repeater used to barely breaking squelch. Now I can't even squelch it out! This will be great for those daily CHP persuits we have here in So-Ca. So far its working good on UHF, Aircraft and 800Mhz also.
I crimped some automotive lugs on the end and screwed it across the beams in the attic.

Here's a pic, (It's with a BNC to F adapter & the antenna is coiled so it would fit in the frame of the picture.)

[basprog]

Wow - nice photo. Glad it's working out for you.

Since the antenna is asymetrical, the directional pattern is anyone's guess (time for me to look into computer modeling). There is also some interaction with the feedline shield. No wonder you don't see these on mountaintops. But it works great in metro areas. At least it acts like a real antenna on the antenna analyzer as compared to some random wire.

There's a lot of room for experimentation by trying to optimize the pattern for your situation by mounting either the short side or the long side as the top element. In addition, one can play with RF chokes (clamp-on ferrites or small coils of feedline acting as an rf choke) placed at the feedpoint or somewhere else down the line.

It's a lot of fun and a definite improvement over the coaxial dipoles!

[oilmillcharlie]

Originally Posted by Uplink

Great antenna Basprog! Thanks! This $5 gatget outperformed my $100 Diamond dipole on Lo-VHF. A local 6-Meter repeater used to barely breaking squelch. Now I can't even squelch it out! This will be great for those daily CHP persuits we have here in So-Ca. So far its working good on UHF, Aircraft and 800Mhz also.
I crimped some automotive lugs on the end and screwed it across the beams in the attic.

Here's a pic, (It's with a BNC to F adapter & the antenna is coiled so it would fit in the frame of the picture.)

What were the lengths that you used for the two legs and what would you recommend for different frequencies. Looks like a great idea.

[Uplink]

Originally Posted by oilmillcharlie

What were the lengths that you used for the two legs and what would you recommend for different frequencies. Looks like a great idea.

I used : "48 inches of wire to one side of a 300-75 ohm balun, and 18 inches to the other side" just like Basprog's post says.


By the way .60 cents at Parts Express, unless your like me and have them lying around in your garage, thinking they were useless until now.

[BenB]

Am I missing something here? Is that all it is for the antenna and it works very well?

Sorry if I overlooked something..this is a new thing to me, and I'm looking at improving, or making my own an antenna..if something as simple as that performs well I'll slap one together with parts I have.

Originally Posted by Uplink

Great antenna Basprog! Thanks! This $5 gatget outperformed my $100 Diamond dipole on Lo-VHF. A local 6-Meter repeater used to barely breaking squelch. Now I can't even squelch it out! This will be great for those daily CHP persuits we have here in So-Ca. So far its working good on UHF, Aircraft and 800Mhz also.
I crimped some automotive lugs on the end and screwed it across the beams in the attic.

Here's a pic, (It's with a BNC to F adapter & the antenna is coiled so it would fit in the frame of the picture.)

[TeRayCodA]

Yes,that is all that is needed.I built a lot of these out of discarded TV antennas a few years ago for fellow scannists around here.You can make quite a few out of an old TV antenna.
They do work quite well.
I ran across plans a few years ago out of a Monitoring Times magazine.The builder was using 1 & 1/2" copper tubing with a pvc "T" as the insulator with another 18" for the mounting pipe.

[FPOWLD]

Are the two wires strung out (horizontically) in the attic and the ends secured to a roof rafter?
What wire did you use? It looks like coax. What type?
Can you get a connector to fit the Balum and put on the cable that leads to the scanner. Otherwise I don't know how to use a jumper or what kind?

Thanks

[TeRayCodA]

Any wire(insulated/uninsulated) will work for the elements.They need to be horizontal.
If you a re going outside with this antenna,get an outdoor 300-75 ohm balun.The balun that is in the pic is an indoor balun.
A run of RG-6 with a "F" connector on one end,and the appropriate connector for your scanner,and you're in business.

[BenB]

I live in an apartment, and currently I'm using a Radioshack 20-032 antenna used for your car. It's great since it doesnt' take up much space, and i can put it on my car when I want.

Would this work better than that? It looks like this could be hidden very well.

[TeRayCodA]

Yes,it will work better.It is physically longer,and can be hidden behind a curtain.

I had two of those radio shack antennas before,and,they always seemed to fall short performance-wise on just about the whole spectrum.The don't work very well if you don't have a good ground plane under them while indoors.

OCFD's are a cinch to make,and offer quite a bit of bandwidth as Basprog stated.

[basprog]

Originally Posted by TeRayCodA

Any wire(insulated/uninsulated) will work for the elements.They need to be horizontal.
If you a re going outside with this antenna,get an outdoor 300-75 ohm balun.The balun that is in the pic is an indoor balun.
A run of RG-6 with a "F" connector on one end,and the appropriate connector for your scanner,and you're in business.

I run the wires vertically and have the feedline jumper coming off at 90 degrees horizontal for a few feet or so before it runs down to the scanner. I'm having better reception with the 18" wire used as the top element.

These indoor baluns aren't too strong, so any light-gauge wire should do. I used #10 gauge and after tinning the leads and few tugs on the antenna, the balun screw mounts came loose so I made another lightweight with common 18 gauge so it wouldn't stress out the terminals.

I'll have to try the other common RS 300:75 balun (part #15-1140B) and see how that works. It appears to be a real isolated transformer inside rather than a transformer (autoformer?) that shares a common ground.

[TeRayCodA]

Yes,for the indoor versions,I had a piece of wood,that I screwed the balun,and the elements into for the stress of stretching it in an attic type installation.(when using a typical indoor balun)
Also,I remember getting aluminum tubing at Lowe's, that would fit into a pvc "T" connector(as an insulator)attaching the outdoor balun at the feedpoint.
Then,you can use the pvc end caps for a professional look on each end of the tubing.

[TeRayCodA]

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!

[Uplink]

Nice diagram TeRayCodA! Gota make one of those outdoor versions.

FPOWLD you can get the adapter (the most "expensive" part this antenna) for the indoor balun here:

F Female to BNC Female Video Adapter



I hung mine in the attic vertically with 5.5 feet of the coax approaching the balun at 90 degrees. Works much better now for our highway patrol (42.34 MHz) broadcasts, as well as the higher bands.