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 2
|Frequency Range||7 - 50 MHz (Too much QRN on 80 and 160m)|
|Area Available||About 9 m x 4 m in the living room|
|Height Available||About 3 m floor to ceiling|
|Height Above Ground||About 30 m above ground level|
|Ground Quality||Probably very poor soil|
|Access to Roof||Unlikely|
|Access to Outside||Through windows only|
As can be seen, there are some advantages to this location, even though it may not be readily apparent at first thought. In order to find the advantages, we may need to be sort of "the eternal optimist." This list may expand as we learn more and think more about the situation, but it is a decent starting point. Our problem now is to design an antenna system that takes advantage of the positive points and minimizes the effect of the disadvantages.
Preliminary Design. Once we understand what advantages and disadvantages we are working with, we can now design a preliminary antenna system that takes account of these constraints. In this example, we know we will need to use a counterpoise or radials with any kind of vertical or will need some sort of dipole so that the RF ground is not an issue. We also know that height above ground will not be something to worry too much about and if we need to adjust the antenna for different bands, it will not be a big chore.
|Center Fed Dipole/Doublet|
When we look at the possibilities, it is apparent that the dipole seems to be the better candidate and should be easy to make and install. The first attempt was to string 65 feet of #14 stranded copper wire outside the windows. It was fed with about 5 feet of coax using an MFJ tuner. The wire was supported by 4 plastic cup hooks glued to the brick on the outside wall of the building within an arm's length of the windows. Since there wasn't enough room to run the 65 ft in straight line, the wire drooped as needed between the hooks. I then had a way to get on the air.
On the air tests showed that the antenna performed quite well on 30 meters and higher frequencies, but was not very good on 40m. Modeling showed that the SWR on the short piece of coax was enormous on most bands and that was compounded by not having very high quality coax. In fact, on some bands the SWR would jump erratically, probably due to beakdown of the dielectric and high voltages in the tuner.
After evaluating the antenna, I decided that using a lower loss transmission line should help. I took down the coax and installed a short piece of 300 ohm TV twin lead from the antenna to the balanced line output of the tuner. The improvement was apparent on all bands and some nice DX was worked on 40m. With the low loss twin lead, the signals seemed to improve by about an S-unit. When you're dealing with a compromise antenna, every little bit helps!