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Case Study 1
Case Study 2
Case Study 3


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ANTENNA NOTES FOR A DUMMY

Restricted Space Antennas

by Walt Fair, Jr., W5ALT

Case Study 1
Travel Antenna for Hotels

Since I often do quite a bit of traveling and stay in hotels, I needed a good small antenna to carry with me. I didn't want to drag an extra suitcase to hold my radio gear, so everything needed to fit in my laptop computer case, along with the computer. I also didn't want to arouse airport security, so it had to be simple and not too bulky. What I came up with is simple, small, has never aroused curiosity and it works with my FT-817 QRP rig. I've used it in Venezuela, Bolivia, Mexico and across the US and had many enjoyable QSOs.

Site Survey. The site survey for this situation is understandably general, since I never know where I will be when I use the antenna. Even so, there are a few constraints I can define, as well as pros and cons of the situation from an antenna standpoint.


ItemComments
Frequency Range7 - 30 MHz
Area AvailableStandard hotel room
Height AvailableAbout 3 m floor to ceiling
Height Above GroundHopefully above the 2nd or 3rd floor
Ground QualityProbably very poor soil, but sometimes close to sea water
Access to RoofNo way
Access to OutsideThrough windows or balcony
Advantages
  • Antennas close to station
    • short feed line (low losses)
    • convenient to adjust (no trip outside)
  • Resonable height
    • lower ground losses
    • no tower, masts needed
    • can lay wires on the floor
Disadvantages
  • Can't use "long" elements
  • Affect of nearby objects (walls, wiring, etc.)
  • No good RF ground
Other Constraints
  • Small and light weight
  • Easily put up and taken down
  • No bulky electronics to attract attention

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:

  • - the tuner was not able to match on 30 and 40 meters
  • - the tuner was a little large to fit easily in my computer case
  • - the tuner and cables did cause airport security to ask questions, although I was never stopped from carrying them.
The preliminary antenna was working and the problems with it had been identified and mostly understood. It was time to figure out how to modify the antenna to get rid of the problems, and hopefully make it work even better.

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.

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