I've always been a fan of wire antennas. At my parents' house, I had 35m of 20A electrical wire strung between the chimneys of two houses, and that was enough to drive an eight ohm speaker from a crystal set. When I started out as an amateur the piece of wire (telephone line I picked up where the technicians discarded it) I had strung out the window and as far up a climbable tree at the Priory in Stellenbosch was hi-Z on 21MHz, the capacitor arced at anything more than 2W output -- but I still managed to work the world.
W4HDX / ZS5L All Band Antenna
This design for an "All Band" dipole fed with two parallel sections of RG-58 coax comes from the January 1993 Radio ZS. Each side of the dipole is 27 feet (8.23 m) long, and so is the feedline. In other words, a reduced length 40m dipole fed like a Spencer dipole.
I used this antenna in the loft of a third-floor flat in Parow, as well as suspended from a short-ish tower, and it seemed to work, although I didn't compare it to something like a G5RV.
So how to get a balanced output from your tuner? Most tuners just use a balun -- specifically, a 4:1 or 1:1 voltage (transformer) balun. These only work well if your antenna is presenting a reasonable load. But the point of course is that your "longest dipole you can fit" is bound to be unreasonable when it comes to providing a load. This is why there's so much effort put into creating abovementioned antennas that present a reasonable load on a number of bands.
VK5AJL says that you should use a current balun since no transformer action takes place, but the currents are balanced and that keeps RF out of the shack.
An alternative is to move the balun to the input side of the transmatch where it always sees a close-to-50-ohm load. This does mean that your transmatch is "floating", i.e. the formerly-grounded side of the capacitor is now at some RF potential -- unless the transmatch is balanced as well.
Another alternative is a link coupled matching network. My 1982 ARRL Handbook, bought
for the then princely sum of R18 from the SARL in
Orphan Street, Cape Town back in 1982 when I was in high school, has this
Link Coupled Matching Network which is rated at
200W. Exactly what I need, but let me tell you, I just can't get this thing to work. My
dipole is 20m per leg, which is pretty much a
80m doublet, and that should be reasonable on at least 80m, right? So Idunno.
Instant Ladderline is a brilliant idea. Spacing is 84mm, and Z = 276log(2D/d) (where D is 84mm and d is the diameter of the wire you're using).
|Wire dia. [mm]||Zc [ohms]|
The only problem is that the impedance is rather high. 30mm spacing would be better IMO.
K0JD reports that he's running 50 watts through a balun consisting of three turns on an Amidon BN-43-3312 core. This core is 19.5 x 9.5 x 25.5 mm with two 5mm holes through it -- tiny by my standards, and I certainly would not have thought that it would handle 50 watts.
hits since 2013-11-15.
|Back to ZS1KE main Page||(This page last modified 2016-08-11)|