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Basic Concepts II
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Vertical Dipoles
Inverted Vee Dipoles
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Ground vs. Radials
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Practical Stuff
General
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 2
Halfway Up a Highrise Condominium

In this case study, I'll explain how I put together a working antenna system inside a condominium in Maracaibo, Venezuela. I had no choice on the location, since my employer paid for lodging. I lived there several years, so the antennas were always an evolving system. During the first 2 years I worked All 50 US States, as well as 155 countries and got enough cards for DXCC. My contacts were on all bands 40m and above and on all modes, including SSB, CW, PSK31, RTTY, MFSK and Hell. In addition, I worked over 125 grid squares on 6m, including several stations in Scandinavia and the former Soviet Union. Most contacts were made with 100 watts, but quite a few were QRP.

Site Survey. Here's a synopsis of the site survey that I made for this antenna installation in cramped quarters. The location is on the 10th floor of a 15 story high rise condominium. There is no balcony and many people told me it looked pretty bleak for putting up any ham antennas. Well, I think they looked at the limitations and forgot that there are some significant advantages, too. As the saying goes, when you get a lemon, make lemonade. But first we need to find out how much of a lemon it is!


ItemComments
Frequency Range7 - 50 MHz (Too much QRN on 80 and 160m)
Area AvailableAbout 9 m x 4 m in the living room
Height AvailableAbout 3 m floor to ceiling
Height Above GroundAbout 30 m above ground level
Ground QualityProbably very poor soil
Access to RoofUnlikely
Access to OutsideThrough windows only
Advantages
  • Antennas close to station
    • short feed line (low losses)
    • convenient to adjust (no trip outside)
  • High above ground
    • 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

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.


AntennaComments
Center Fed Dipole/Doublet
  • Doesn't need an RF ground
  • Can use entire horizontal space
  • Can hang outside the windows
GP Vertical
  • Doesn't need an RF ground
  • Will be very short
  • Hard to install radials

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!

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