For years, I’ve been wanting to put a VHF/UHF dual-bander in my home office. Two things have stopped me from doing it – outside egress and not wanting to mount another antenna to the roof. However, a few months ago I won a participation door prize from a radiosport contest to HRO. It was just the perfect amount to purchase a Comet CTC-50M Window Gap Jumper which I have always wanted to play around with. The goal in that purchase was to evaluate it for recommendation to people who need to get an antenna up and no good option for getting a feedline out of the house.
After purchasing it, the jumper sat near the bench for a few weeks until I had an epiphany – I would use the jumper to install a radio in my home office AND I’d construct a mount for clamping onto a gutter using another part I’d excavated from “the bench” during a clean-up and organizing spree. The idea was to use common, low-cost parts to help people who have HOA restrictions, are renters, or desire low-visibility characteristics put up an removable, no-“damage” antenna that wouldn’t be noticed or, if it was, people wouldn’t be bothered with it. That could be fed (or not) with the window passthrough. The other thing I’d excavated from a pile of randomness was a CB radio antenna clamp-mount for trucks I’d purchased as one of the options when I was installing the mobile in my truck but never used. More about that part in a few paragraphs.
The window jumper simply works as advertised. It’s a very thin feedline encased in a ribbon dielectric. It’s designed for “soft seal” pass-through such as a window sill or a door with a weather strip – i.e. situations where there’s some give and compression in the thing being closed upon the jumper. Each end is an SO-239. The insertion loss under 500 MHz is less than 0.5dB and it can support up to 60W on 2m and 40W on 70cm. Put it in place, secure the cable so it doesn’t bunch up under the window or door and then that’s pretty much all there is to it. It just works as advertised.
For the antenna, I decided to use the CB radio clamp on the rain gutters to mount an antenna. The idea was the gutter would serve as a ground plane to a mobile-style UHF mounted antenna. The design would sandwich an SO-239 barrel and the radio mount clamp between the feedline and the antenna. The opening in the CB mount was designed for the standard CB-sized antenna mount assembly that mounts a screw-in 3/8″ antenna. Using a Dremel with metal cutting bit, I enlarged the hole in the clamp so that an SO-239 barrel would pass through.
After carefully removing a small amount of paint under the fingers of the clamp to get a solid electrical contact, I screwed the mount onto the gutter. It tightens up quite nicely without crushing the aluminum of the gutter. Then I screwed the antenna onto the barrel, added a washer between the antenna and the mount plate, slid it into place with two washers between the plate and the feed line, and then attached the feed line.
Overall the system worked great. I used a Comet SBB-14 that I had on hand and the gutter formed a nice ground plane for the antenna on 2m and 70cm. See the SWR plots. Obviously the SBB-14 isn’t exactly “low profile” as it’s a tri-bander with 6m capabilities. However the system would work just as well with a low-profile antenna such as a Comet SS-680SB, Comet SBB-2, or a Diamond SG7000A – all of which are less than 19” tall. An all-black antenna is suggested for low-viz over chrome-plated ones.
The full parts list, besides the radio and standard feedline is:
- Comet CTC-50M ($60)
- Workman QRCS3 ($22)
- SO-239 barrel connector (example only; used a generic one I had on hand)
- 3 x 5/8-in stainless steel flat washer