I've recently received a lot of question regarding my portable self supporting dipole after having a photo of my station appear in the June 2021 edition of QST magazine. I submitted the photo in addition to my logs for the 2020 ARRL Sweepstakes Phone contest.
Here is the original I submitted. I was operating at Santa Teresa County Park in San Jose, CA.
Here's the dipole at the same park on a different day. I made RTTY contacts with stations in Finland and the Philippines.
Here I pulled over on the side of the road near Lake Anderson in Morgan Hill, CA. This time I got the mast extended higher than previously. I worked a station in Japan and another in Chile on SSB.
Here in northern California I have plenty of trees to place a dipole or end fed half-wave. Additionally I have a 5 meter tall telescopic whip I can mount on the top of my car to get on any band between 20m - 2m (this is not always my first choice because verticals have their issues). However, there are some locations I operate from where I don't have trees (Mojave desert) or park restrictions prevent using them. For these cases I've settled on this self-supporting 20m - 2m dipole.
- MFJ-1906H Mast
- 10 m (33 ft) tall fiberglass mast
- 5.8 kg (13 lb) weight
- Pretty rigid and sturdy
- Made of 6 sections
- 2 in bottom section
- 3/4 in top section
- 2x MFJ-1979 Telescopic Whips
- Extend to 5 meters (1/4 wavelength on 20 meters)
- Can be collapsed to cover down to less than a meter
- Using two can make a 1/2 wavelength dipole for everything between and including 20m - 2m
- 3/8-24 threading
- MFJ-347 Dipole Adapter
- 3/8-24 threading
- Mounts up to 11/4 outer diameter
- 15.25 m (50 ft) MPD Digital Genuine Times Microwave LMR-195
- 6 turns at a diameter of about 27 cm (10.5 in) to make a choke. Coil held together with zip ties
- Tire Mount for Large Diameter Flag Pole (Link)
- A little larger diameter than the mast but fits well enough
After acquiring all the materials the first step is to place attach the MFJ-347 dipole adapter to the top section of the mast. Afterwards I took one side of the coax and looped it around 6 times with a diameter of 27 cm. I then used zip ties to keep the loop in place. This loop acts as a choke keeping RF from coming down the coax and interfering with the radio/computer. Finally, I'd use a marker to mark each section of the mast 30 cm from the bottom. This way as I'm lifting each section I know when to stop and clamp. Without this I might lift a section to far and have it fall out. This is pretty much all that needs to be done until I deploy it.
Once I'm ready to deploy I drive my back tire over the tire mast support and insert the mast into the support. I then attach the coax to the MFJ-347 and screw on a MFJ-1979 telescopic whip on each side. I extend the whips all the way out (I usually use this on 20 meters). The mast has six sections so I unbuckle each clamp and raise the section until I reach the mark I mentioned earlier. At this point I'm ready to operate.
The tire mast support:
Notice the coax is looped several times to make a choke. I learned through trial and error that without this too much RF would come back down the coax and into my radio/computer.
The results are quite impressive. The antenna is a full size dipole and therefore resonant on any band between 20m - 2m when the whips are folded/extended to the right size. The mast can go to 10 meters making it a half-wavelength high on 20 meters. This gives a very good angle of radiation. I've been able to work lots of DX using this antenna. I'm very happy with the actual RX/TX performance.
In terms on rigidity and stability, the antenna does pretty good so far. The telescopic whips bend a little under their own weight but that hasn't been a problem yet. I'll know more as I deploy this antenna more often and see if/where the stress breaks anything. I haven't operated this antenna in heavy wind but I'm confident that any moderate day in California is fine for this antenna. As I'll mention in the next section, I plan on trying some guy rings to strap the mast down. This will be especially important to handle the wind I anticipate experiencing when I go out the Mojave desert again later this year.
Though I'm very happy with this antenna, a few improvements come to mind. The first of which is getting some guy rings and rope in order to strap this antenna down. It's very stable as it is, but it'll make me feel a lot better. The second is doing something to relieve the strain the loop of coax (choke) places on the SO-239 end that attaches at a 90 degree angle to the mast. I feel this hasn't been a problem but I worry that connector may give out one day.