[Cliffdwellers] limited space antenna

[Terry Conboy]

On 2009-03-01 11:51 AM, Hart VA3KIS wrote:
> I live in an apartment in a house on the third floor. Were I currently have
> a butternut vertical mounted on the roof. The houses are in my area are
> known as "row houses" because they are so close together. And the back yard
> is smaller in length of a mid size car. At the back of the house I have a
> fire escape, which I use to enter my apartment. I have a balcony that is
> built on the roof of the house which is about 10' by 15' which I enter my
> apartment through.  I'm not happy with my current vertical antenna and  I
> wish that there was a way to design a low key but affective wire antenna for
> all of the HF bands [80 - 15M].  The reason for my wanting a wire antenna is
> that this vertical is very noisy especially when there is a subway line
> running under the house.  I currently have a ft-2000 with a mfj-989d roller
> inductor tuner. So tuning the antenna will not be a problem.  So...  I
> thought by sending a note on this reflector that there would be someone out
> there in ham land who could help me design a wire antenna for my current
> situation.
>   
Hart,
This is a tough situation.  I don't think there is any cut-and-paste solution to fitting a wire antenna into your real estate.  Unfortunately, it will probably be a matter of trial and error to see if you can find an antenna configuration that picks up less of your local noise than your existing vertical.  I'm sure you will get recommendations to use some sort of closed loop antenna since they have a reputation for being "quiet", but in reality, this is mostly an old wives tale or wishful thinking.  Loops can be just as noisy as dipoles and verticals.

You didn't say what you were using for ground at the vertical, but this can often be very important in noise pickup.  One easy thing to try is to insert a current (choke) balun at the feedpoint so the coax shield is isolated from ground.  This may or may not help, depending on where the noise currents are flowing.  In addition, along with the choke balun, it can be a good idea to use tuned radials on your vertical that are insulated from the building.  This will float your entire antenna system and can sometimes reduce ground loops that inject noise into your feed system.

Your roof is probably too small to stretch out 80m or even 40m radials, but you can bend them somewhat and extend them over the edge of the roof and drop them straight toward ground or pulled away from the building.  Make sure they are insulated from the building and the earth, especially at the ends where the voltages can be pretty high with even a medium power TX.

In many situations, horizontally polarized antennas will pick up less noise than a vertical, no matter how clean the ground system is on the vertical.  Your three story building is high enough to get a decent takeoff angle on 20m and 15m, and you can do reasonably well on 40m.  On 80m, plan for mostly short haul QSOs.  I'm not sure how large your roof area is, but you can run a bent "dipole" around the edges of the roof, bringing the ends near each other and feeding at the midpoint with open wire line to your tuner.  You need to shoot for a total of about 88 feet of wire for 80m at a bare minimum.  The classic G5RV length of 105 feet is more efficient on 80m.  The exact length isn't critical, and more wire, up to around 1/2 wavelength on 80m is better, but it may be hard to handle ~134 feet of wire in your location.  Don't be afraid to bend the wire to follow the edges of the roof.  The less it doubles back the better, but do what you need to fit the wire in your space.  It is also acceptable to drop the ends of the wires over the edge of the roof, but more vertical wire may mean more noise pickup.  You just have to be creative and experiment a bit.

Using small wire, down to #26, if you don't get too much ice, can be a good way to stretch your horizontal antenna out away from the building if you can find some support for the far end.  I like bare wire (no enamel) since it oxidizes and the dull surface doesn't reflect light very well.  Just be prepared to replace it when (not if) it gets broken.

There's nothing wrong with a horizontal loop antenna (such as the "Skywire"), but it generally needs twice the length of wire as  an open ended antenna to get reasonable feed impedances and efficiency.  A loop with a one-half wavelength circumference on your lowest band will work, but the feed impedance is extremely high (> 100Kohms) and the efficiency is marginal with small diameter wire conductors, so something like 5/8 wavelength circumference is probably a minimum (about 175 feet on 80m).

Let us know how it goes.  Good luck!

73, Terry N6RY