Basic Concepts I
Basic Concepts II
Inverted Vee Dipoles
Ground Plane Verticals
1/4 Wave GP Verticals
Ground vs. Radials
Case Study 1
Case Study 2
Case Study 3
ANTENNA NOTES FOR A DUMMY
Restricted Space Antennasby Walt Fair, Jr., W5ALT
Case Study 1
|Frequency Range||7 - 30 MHz|
|Area Available||Standard hotel room|
|Height Available||About 3 m floor to ceiling|
|Height Above Ground||Hopefully above the 2nd or 3rd floor|
|Ground Quality||Probably very poor soil, but sometimes close to sea water|
|Access to Roof||No way|
|Access to Outside||Through windows or balcony|
As can be seen, there are some advantages to this situation, even though it may not be readily apparent at first thought. I can usually request a hotel room on the top floor or as high up as possible. Of course the disadvantages are the size constraints, the unknown ground quality and the effect of nearby objects that changes with every trip.
Preliminary Design. My first try was to use a Radio Shack reel antenna (normally sold for SWL use) along with a small MFJ tuner. The reel antenna contains about 19 feet of stranded #20 wire in a small case that allows it to be reeled in when not in use. The antenna wire was connected as a random wire and worked well on 20m and higher frequencies. I made quite a few QRP contacts using that system.
There were, however, 3 problems that still needed to be overcome:
Modified Design. As can be seen, all of the problems stemmed from using the tuner. If I could eliminate the tuner, then the system would work according to my needs. So, I got out my antenna modeling software and decided to figure out why the antenna wouldn't work on 30 and 40m and also see if there was a way to get rid of the tuner.
Modeling the antenna at 40m showed that an end-fed wire presented an extremely high impedance , explaining why the tuner wouldn't work. I decided to see if I could add a counterpoise to help get it to tune. While in the process of playing with the model, I discovered that if I used 2 wires it was fairly easy to get an exact match to 50 ohms. The modeling indicated that with 1 wire about 1/3 wavelength long and the other about 1/6 wavelength long, if the angle between the wires was around 45o, a perfect match could be attained. That meant there would be no need for a tuner at all.
The next step was to try it. I measured out 1/3 wavelength for 15 meters and connected the wire to the coax center conductor of the FT-817 and clipped the other end to some curtains. Next I measued out 1/6 wavelength and alligator clipped that to the coax outer shield connector and laid the wire on the floor. Sure enough, a slight length adjustment on the shorter wire and the SWR was 1:1, just as predicted. I then repeated the experiment on 20 meters and 30 meters with success. Unfortuantely the wires were too short to get 1/3 wavelength on 40 meters, but with both reels fully extended, the SWR was acceptable on 40m, too.
As a result, I now do not carry the tuner with me at all and have never been asked anything by airport security. The entire antenna system consists of 2 Radio Shack reel antennas that are small enough to fit in my shirt pocket when not in use. There is no feed line or tuner, so losses are small. Modeling indicates the antenna has some gain broadside to the wires. Of course, this is still a compromise antenna, but I have been able to make many contacts from hotel rooms without much problem. The design now fulfills my requirements, but I reserve the right to modify it later if I see a better way to do things.