|
|
|
|
The Coax fed Fan
Di-Pole, Lot's of Bang for a few Bucks. The Coax Fed Fan Di-Pole can get you on the air over 4 different bands with a single coax penetration into the shack. Even better, there is no need for a tuner or the signal loss that a tuner implies. How does this work and what bands are you talking about?That's usually the next question I've been asked when I mention this antenna. Well, how about 80m, 40m, 20m, 10m for the bands? As for how this thing works...RF will always take the path of least resistance (impedance). What does this mean? Well if you have 2 di-poles connected to the same feed, one cut for 80m and the other cut for 40m and when RF is applied to the feed point at a frequency of 3.8MHZ (80m), the 40m di-pole is too short to provide a good match and so presents very high impedance to the feed point. But the 80m di-pole connected the the same feed point is cut to provide a very good match for the RF at the feed point. The RF "chooses" the 80m di-pole pathway when the frequency is 3.8MHZ. Let me say that again as it's the core principal here. The RF "chooses" the correct di-pole based on the frequency of the RF. Think about that for a moment. I'll wait......... You want to make one, you say? OK. Here is a simple plan to do just that. First, You need to determine how long the longest wire will
be on 80m, so you know how to hang the antenna on your property.
468 / Frequency in MHZ = Overall length of di-pole in feet This will give you the OVERALL length of the di-pole. I used
a frequency of 3.9MHZ which is the center of the SSB voice (phone) General
spectrum in the band plan.
Save some cash: That's a total of 254 feet of wire! It's going to cost a lot of cash! Nope. The di-poles can be made out of inexpensive 18 gauge stranded wire. Go to your local Lowes, Home Depot, or home improvement center and look in the electrical/outdoor section.You want to find radio-dog fences. Yes. Radio-Dog Fencing. The wire is usually sold in a 500 foot spool and guess what? It's 18 gauge stranded wire intended for burial. I bought mine a few years ago for $19.98, I last saw it a month ago for $22.89. Go buy two, you'll thank me later. (Or use your preference of wire) Now, we have our wire. We'll need some Coax to feed this guy. I used some RG-8X that my local Radio shack had laying around in the back. The 8X is much thinner and lighter than the usual RG-8 and is easier to work with for me. I highly recommend it for this application. There is a slightly higher loss with it compared with some of the more expensive types, but I don't think you're going to miss a watt or two, if it means you don't have to wrestle with the Coax. Make sure you get a PL-259 or two while you're out, with the proper adapter for 8X. There is just one more component needed and to get this one, you will have to make some coffee. Huh? Yep. Coffee. It's sold now in 15oz (1lb is gone in my area) round plastic containers with a convenient lid that snaps on and off and is pretty much air-tight. Go get one. Empty it however you want but remember, we will be soldering later so don't drink it all in one sitting if you want to build this antenna today. OK. On to the build. The feed point is constructed with the coffee can using it
as the housing and common-mode choke. What's a common-mode choke? I will
explain but I strongly suggest that you look here: http://www.hamuniverse.com/balun.html
A common-mode choke is also called a 1:1 Balun, Ugly Balun,
Choke Balun, Air Balun, etc. It is used to eliminate currents that can
develop on the outside of the coax shield when the coax is used to feed a
balanced antenna. The
di-poles. If the two legs of your di-pole are not EXACTLY the same length, you will drive yourself crazy trying to tune it. Let me say that again: If the two legs of your di-pole are not EXACTLY the same length, you will drive yourself crazy trying to tune it. Here is a good method to ensure that this does not happen to you. First, cut the dipole for the "Total Length" in the chart above. Then, fold that wire in half, tie the ends to something solid and walk away while holding the wires. You will end up with a loop at the end in your hand. Cut that loop at it's apex. Go back to the spot where you tied off the wire and measure out each wire to the "Ideal Length" below. Once you have the proper length, twist the remaining wire back around the standing portion making a loop. The apex of this loop should be at exactly your measurement.
Final
construction, Lets bring it all together. Find a drill bit that is just large enough for your 18 gauge
wire to pass through the hole it makes. Since the longest wire is the 80m di-pole, it will provide
the support for the entire antenna. Strip all the legs and connect one set of legs to the center
conductor of the coax, solder it well. Di-pole
supports. You will need some supporting material to keep the legs of
the di-poles from tangling together when its in the air. The easiest thing
to use I've found is hardwood dowels. Tuning. Tune the dipole by checking SWR on the 80m Di-pole and making adjustments by un-winding the extra wire on the end and reforming the loop. Do this the same amount on each side of the di-pole.Only once the 80m is done can you move on to the 40m, 20m, and 10m. Once each antenna is tuned to your satisfaction, go back and re-check each one. It's best to develop a chart to show the frequency where the SWR "dips", I even plot mine on graph paper. What can I say? I'm visual. This makes it easy to determine your best operational frequency ranges and provides a reference for spot checking the antenna later. When you are done tuning, send that fan di-pole into the air as high as you are able and make some contacts! 73 - KB3PKB Editors note: The multiband fan dipole is probally one of the most
efficient multiband antennas you can build due to the fact that individual
dipoles are tuned for each band of operation and fed by one single length
of coaxial cable. The end result is "one antenna" for multiple bands and a
single feedline. Questions? Email Tom here: KB3PKB at gmail.com
 Hamuniverse.com uses Green Geeks Web Hosting! |
