Musings on Antennas, Tuners and Baluns

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.

Photo of mast in the back yard at 15th Avenue

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.

The idea is to match the feedpoint impedance to the 50 ohm load using a specific length of transmission line, and that by carefully choosing the length of the dipole and the length of the transmission line, you can make this happen on more than one frequency.

But it does seem a lot simpler to "Put up the longest dipole you can fit, feed it with open wire line, connect it to the balanced output of your tuner and poof! Instant multiband antenna".

ZS5L article from Radio ZS

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 130ft to 140ft 80m doublet, and that should be reasonable on at least 80m, right? So Idunno.

Strangely enough, the Norcal BLT, which is sort of a Z-Match, seems to work well. It can't take more than a few watts though, with those plastic variable capacitors. Another Z-Match

Instant Ladderline

Three packets of instantladderline spreaders

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]
(This is the table from the Wayback Machine copy of, with values rounded to a sane figure. Home-made ladderline impedance to two decimals? YHGTBSM).

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.

I'll just leave this here

Wire Guages
awg swg mm awg swg mm
- 47 0.05 24 25 0.5
38 42 0.1 23 23 0.6
35 38 0.15 21 22 0.7
32 36 0.2 20 21 0.8
30 33 0.25 19 20 0.9
29 31 0.3 18 19 1.0
27 29 0.35 12 14 2.0
26 27 0.4 9 11 3.0
25 26 0.45 6 8 4.0

[Image] Hit Count hits since 2013-11-15.

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