Folded Monopole for 440 MHz

Here's a simple, low cost 440 antenna for those of us with vans who barely have clearence in standard parking garages and have no problems drilling through the roof. Having its own radials (rather than relying on good contact with a roof that may rust eventually anyway), it could also be used with non-metallic surfaces.

Noting that a simple 1/4 wave over a groundplane has 30-35 ohms impedence, an asymmetric folded monopole is used to bring the impedence up to 52 ohms and to widen the bandwidth of the antenna. I currently only have 5W into a Radio Shack 144/440 SWR meter, but it appears to be 1:1 at 445 MHz and the needle barely moves at 440.25 and 449.20 MHz. So there appears to be at least a decent impendance match.

Of course, it's not a gain antenna, but it's cheap and serves my purposes well, given that part of my current commute is through somewhat mountainous terrain where the "pancake pattern" of a gain antenna isn't necessary helpful. But it is a simple low-profile antenna that should out perform a 'rubber duck' and may solve some people's clearance problems.

Materials:

Female chassis-mount BNC or N connector.
Solder lug to fit connector (if not supplied)
Rubber washer or gasket (to fit BNC or N connector)
6 1/8" of 5/32 brass tubing (hobby shop)
6 1/2" #22 insulated wire (to allow extra for soldering)
About 2-3" of 1/8" copper refrigeration tubing [optional, see text]
1" of brass rod or copper wire to fit inside copper tubing [optional]
3 or 4 wire radials, 6 3/8" each (diameter/type non-critical)
Sealant

Side view:

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	  |  | |  6 1/8" of 5/16 tubing with
	  |  | |  ~6 1/2" of #22 insulated wire
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	  |  | |     6 3/8" radials (3 or 4)
	  |  | |_______________________________________      (To rear)
	  /  \  |
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 	=|    |=|
=======================================================================
	 |    |
        =|    |=  BNC (or N) connector
	 |____|

Top view:

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      ..........O+-----------	(...... optional)
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Instructions:

Drill hole in roof according to the size of your coax connector. Using a rubber gasket (or washer or some other device to prevent leakage), followed by solder lug, install connector with the solder side facing the sky. Solder #22 wire to the top of the 5/32" tubing. Securely attach 5/32" brass tubing to BNC or N connector. (If you use 1/8" refrigeration tubing, it will fit nicely inside the tubing, so you can adjust the height of the antenna. You can solder a short piece of wire to the 1/8" tubing and that to the BNC.) At this point, you might want to check for shorts, as a folded monopole has zero DC resistance. (I learned the hard way.) Bend the solder lug as close as you feel comfortable with to the connector, and solder the three or four radials to it. Droop them slightly so they lie across the roof.

Now, if you've used nested tubing and you want to tune this rather than relying on measurements given above, use an alligator clip to hold the other end of the (end stripped) #22 wire. Dress the #22 wire as close as possible to the tubing. Take 3-5 measurements across the the band and adjust the length 1:1 SWR (or close to it) mid-band. If you're sure about your adjustment and don't expect to change anything, then solder the tubing together so you will always be assured of a good RF connection.

Then, solder the #22 wire to the solder lug. Apply sealant at relevant points on the connector (including at the solder point on the BNC, particularly if you can twist the tubing), and at the top of the tubing, so rain doesn't get inside (unless you've already sealed it with solder). You're done.

-- KD6PAG ("Networking old-timer, RF newbie")