Case Study 3
Indoor Multiband Vertical
In this case study, I decided that it would be nice to see how a short vertical antenna worked inside the condominium in the previous example. The site conditions are the same, so will not be repeated here.
Preliminary Design. The preliminary design was driven more by physical constraints than by antenna theory. I bought 3 meters of aluminum curtain rod tubing from a hardware store along with 10 meters of 3 conductor solid copper house wiring. After taking the plastic jacket off of the 3 wire bunde, I ended up with 3 10 meter lengths of wire. Two of these were laid along the baseboards as radials as far as they would go without crossing doors, etc. The 3rd wire was used to wind a loading coil on a 5 inch plastic waste basket. The aluminum was mounted on top of a toilet brush holder that served as a support for the antenna. With the antenna in a corner of the room, it was out of the way. About 25 ft of coax was used to feed the antenna from my MFJ tuner. The center coax conductor was attached to an alligator clip to select a coil tap, while the shield was connected to the radials.
Initial tests showed that the antenna worked quite well on 6, 10, 12, 15 and 17 meters. On those bands it was as least as good as my doublet and on 10, 12 and 15 meters it appeared to be slightly better. I could tune it on 30 and even 40 meters, but the signals were substantially worse that the doublet. I used the antenna like that for about a year and worked lots of DX.
While thinkng about the antenna and how to improve on it, I realized that the 25 ft of poor quality coax wasn't helping anything, especially since the SWR was pretty bad on the lower frequencies. After doing some modeling, it seemed that the perfomance depended more on the transmission line SWR than on the actual antenna radiation characteristics. In addition, the coax was laying very close to one of the radials, so that didn't seem to help the situation either.
In order to try and improve things, I got rid of the loading coil and installed a remote antenna tuner at the base of the vertical. In addition, I added a coil of coax to form a choke balun to try to reduce the feed line current. The improvement was immediately obvious, with signals improving by about 5 - 6 dB on receive. My signal reports also improved correspondingly. The antenna now performs at about as good as the doublet on 20 and 30m and at times gives much lower noise on receive on 40 meters, too.