I have three antennas for the station — an off-center-fed dipole for HF; a dual-band vertical for VHF-UHF FM; and, another dual-band vertical for my APRS station operating on 144.39 Mhz.
Off-Center Fed Dipole (OCFD)
This antenna is cut for 80, 40, 20, and 10 meters. OCF dipoles are resonant on EVEN multiples of their lowest frequency. Mine is cut for 80 meters, so, it’s resonant on even multiples of 3.6 Mhz: 7.2, 14.4, 28.8.
The dipole uses a 4:1 balun that has a 1:1 balun built into the same case. I feed the antenna with RG-213 coax.
The balun was purchased from Balun Designs — found here: http://www.balundesigns.com
Here’s a picture of the balun.
Inside the box is a 1.5 KW 1:1 balun that isolates the coax from the antenna and then feeds the 4:1 balun also in the box. The eyelet on the top is used if you hang the balun; the eyelets on the sides are used to connect the two legs of the antenna; coax connector is on the bottom.
Here’s a close-up picture of the balun mounted on one of the plumbing vent pipes on my roof, with the two legs of the antenna coming off the balun in a V shape. The balun is mounted onto a piece of 1/4-inch aluminum stock which clamps to the vent pipe with two U-bolts. The two legs of the dipole tie to the two side eyelets on the balun — one leg to each eyelet. Coaxial cable feed line comes out of the bottom of the balun. I use UV-resistant nylon zip ties to secure the cable to the vent pipe and provide slack so the cable does not hang with its full weight on the balun or on the end of the vertical antenna.
And here is the layout of the antenna.
SHED ROOF—rope—insulator— 83 ft 2-1/2 inches wire –BALUN— 46 ft 9-1/2 inches wire —insulator–rope–MAST BY GARAGE
The coaxial cable feed line to/from my rigs attaches to the BALUN.
Two ends of the OCF dipole
This antenna is not very high. As you see in the photo, the balun is mounted onto one of the PVC vent pipes on my roof, about 25 feet off the ground. One leg of the antenna (the 83-foot length) goes to the roof of my workshop/shed where it terminates about 15 feet off the ground. The other leg runs to a vertical post made of pressure-treated lumber beside my garage where it terminates only 10 feet off the ground.
How does this antenna work? Here’s my experience:
80/75 meters: Because the antenna is so low, it is a bit tricky to tune on 80 meters. With the MFJ tuner I am able to get the SWR at the rig down to between 1.5:1 and 2:1 across all but the lowest and highest ends of 80 meters — I can’t operate below 3.575 or above 3.900. I believe if I got the antenna above about 35 feet high it would cover all of 80/75 — but I have no way of getting that much height.
40 meters: The antenna works well across the entire 40-meter band with SWR 1:1 across the band.
20 meters: Works well across all of 20 meters with SWR between 1.5:1 and 2.5:1.
10 meters: Works well across all of 10 with SWR from 1:1 up to 2:1.
Now — the proof of an antenna system is NOT the SWR, but, how well it gets a signal into the air and how well it receives. I put up this antenna in November 2008 and so far have been pleased with my contacts.
I am not a “DX Big Gun.” I operate almost entirely QRP CW, mainly with my Yaesu FT-817 at 5 watts or less. Sometimes I dig out one of my many single-band QRP rigs running from 0.5 to 2.5 watts. I’m happy with the antenna and I think it was a good choice for my limited space and height.
From the time I installed the antenna until fall 2011, I worked mostly 40 meters QRP CW into Europe, South America, most of the US, Canada, and Mexico — no luck into the Pacific or Asia. Now, in early 2012 with band conditions improving worldwide, I’m looking forward to seeing what this simple antenna will do for me.
I tried a couple of other antennas — an 80-15 meter trap dipole, and an 80-15 fan dipole. Had problems with both of these; couldn’t get the SWR down on some bands.
I may add to this antenna a 135-foot dipole fed with open-wire line — I could install that antenna in the same manner as this one — center insulator at the same PVC vent pipe, ends to the shed and garage — although it would be a bit tricky getting the open-wire line from the antenna tuner out the shack window. Also, I’d have to add a manual antenna tuner with balanced output.
My other antennas are a Diamond Model XC-30A fiberglass dual-band verticals for 144 and 450 MHz fed with RG-9913 coax. One of these is for my FTM100DR VHF/UHF FM transceiver and the other is the UHF/VHF antenna for the FT7900 I use to monitor local public service frequencies.