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2M folded dipole

Sep 9th 2011, 01:59

KC6SDF

Joined: Aug 22nd 2011, 00:51
Total Topics: 0
Total Posts: 0
Hello,

I am interested in putting up a folded dipole on my mast, I am running simplex. I browsed my HAM shop and found that they are extremely expensive. I would appreciate help with some questions...

1). What is their advantage/disadvantage?

2). Are there any construction plans with formulas to apply to different frequencies? I am feeding my current vertical antenna with LMR-600 50ohm coax.

3). I am picking up intermod with my Cushcraft Ringo Ranger 2 and IC-2200H. What do you recommend? I need to be able to tune it.

Best Regards,
Nick

Sep 9th 2011, 04:14

KM3F

Joined: Mar 6th 2008, 13:50
Total Topics: 0
Total Posts: 0
Hello Nick,
Answer to #1, There is little to no advantage to this configuration.
Reason is it has a 300 ohm impedence and will present a high SWR. The ratio would be close to 6 to 1. 300/50 = 6
No performance advantage over a dipole unless you proceed to make a beam with some gain and use some sort of impedence matching system from 300 to 50 ohms coax.
If you still want to make one, just use some television twin lead cut to length using the info in #2. Short the ends and open "one" side in the center. Mount it to a thin wood stick.
#2. A formula to, try 468/ f. Example 468/ 146.5 mhz.= 3.2 feet rounded off. Take 3.2 x 12 inches and you get 38.33 inches for length, then trim the length to offer a broad match over the frequency bandwidth of interest..
It is much easier to make a dipole that will come very close to matching your 50 ohm coax. It's natural feed impedence is about 72 ohms +/-.
#3. I can't address because I don't know what or where your are getting if from or even if it is inter-modulation.
Hope this info gives you some help.
Good luck.
Sep 9th 2011, 11:24

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Repeaters often use arrays of folded dipoles because it is one of the most convenient ways to get an omnidirectional pattern when you don't have access to the top of a tower. The best tower locations go to the customers who will pay the most. Properly designed and constructed, they are quite resistant to high winds. It isn't unusual for fiberglass whip antennas to have issues at windy repeater locations.

http://www.repeater-builder.com/antenna/ant-sys-index.html#antennas
This site has lots of documentation on commercial quality folded dipoles

The IC-2200 isn't known for being intermod prone, but given adverse conditions, any radio can have issues. It is also possible that the intermod is being generated outside the radio--this can happen at the antenna (rusty joints) or at a busy repeater site. You can test with a step attenuator--the intermod products will fall much faster than the real signals as you reduce signal strenths. If it is the radio, you may be able to cure the interference by notching out one of the strong signals causing the interference. This can be done in the frequency domain with filters or by altering your antenna pattern. Another test is to put a tight bandpass filter ahead of your radio--often one of the interfering stations isn't even in the ham bands.

Zack Lau W1VT
ARRL Senior Laboratory Engineer

Sep 12th 2011, 00:34

AA6E

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
The main electrical difference between a simple dipole and a folded dipole at resonance is the feed point impedance. (Factor of 4 higher*). The FD should have a wider bandwidth (e.g., at VSWR 1.5:1 or 2:1), because the 2 conductors act like a fat simple dipole. There might some mechanical advantages for mounting, and it's certainly convenient if you need to use 300 ohm balanced line for for some reason, e.g., because 300 ohm "twin-lead" has low loss compared to coax. I can't think of why an FD should be much more expensive to make than any other dipole design.

* You can get other ratios besides 4:1 by making one of the dipole conductors with greater or smaller diameter than the other, or by using 3 parallel conductors, while feeding only one, etc.

73, Martin AA6E

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