For the fun of it, and with some time on my hands, I decided to try out some mini vertical antennas for the 20 m band:
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28-May-08. This antenna is simply my 20 meter mini-dipole installed in a vertical position. Note that each of the two fishing poles that make up the arms of the dipole, is almost 3 meters long. I used a 3 m section of 40 mm diam. PVC pipe from the DYI store as a mast. The base of a large room fan served as a temporary stand.
My
vertical mini-dipole. Crossing in front of it is half of my
Cobra folded dipole.
I only tested the antenna very briefly, as there was too much wind for the antenna to stay upright by itself. Received signals seemed similar to those of my Cobra folded dipole (multi-band) antenna. Will have to do more extensive testing, preferable with a constant test signal. To be continued...
The coax cable and connector are
attached to the mast with standard tool clips
2-July-08: for fun, I experimented with half of the centre-loaded vertical dipole as a
mini-vertical. Installed it about 1 meter of the ground. As a radial, I used
3½ meter wire (just because I had it handy), running horizontally from the base of the antenna. Tunes easily.
Marc, DL5NA, gave me an SSTV RSV of 585. Some stations come in slight weaker
than with my Cobra dipole, others same strength. Below is the plot from my
miniVNA antenna analyzer (antenna + coax + choke balun). The resonance peak has
moved up a couple of 100 kHz with respect to the mini-dipole. More to experiment
with!
Note: a low SWR (indicated by the antenna analyzer without ATU) doesn't necessarily mean that the antenna is working well! In general, short vertical antennas need a a good ground/counterpoise. If not, the loss resistance of the counterpoise is (very) high with respect to the radiation resistance of the antenna. This loss resistance adds to the (desirable) radiation resistance of the antenna, and the antenna efficiency E goes way down: E = Rradiation/(Rradiation + Rloss)
Update 5-August-08: I have built-up this antenna once more. This time higher up, like the slinky-coil loaded vertical discussed below.
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once with a single radial of 3½ meters (10 ft), sloping down at about 30 degrees. In this configuration, the resonance frequency went up to 14.770 MHz (from 14.140). Radial too short (much less than ¼ λ). |
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once with that radial wire sloping up and connected to the rain gutter of my apartment. This metal gutter partially wraps around my apartment and that of the neighbors, and is about 50-60 meters (150-200 ft) long. In this configuration, the resonance frequency went down to 12 MHz (from 14.140). Too much capacitance! |
14-July-08. This antenna is simply the top half of the center-loaded vertical mini-dipole shown above, loaded with a Slinky coil counterpoise. Call it an end-fed antenna, whatever.
The Slinky coil is slid over 2 meters of PVC mast (from my mini-Yagi); it hangs down from where the monopole is attached to the mast. I connected the center conductor of my standard 12 meters of coax to the bottom of the mono-pole, and the braid/shield to the top of the Slinky coil. As always, a W2DU current choke "balun" at the other end of the coax.
First I tried to tune the antenna by shorting-out a number of windings at the bottom of the Slinky coil. Managed to tune from 8-12 MHz and above 15 MHz, but not in the 20 meter band!? Gave up on that, and tried to tune simply by raising and lowering the bottom of the Slinky coil. With a free-hanging Slinky coil, the resonance frequency was about 12320 kHz, resistance very close to 50 ohm, and an SWR of 1.2. See the he antenna analyzer plot below (left). Not bad, just the wrong frequency, hi! Raising the bottom of the coil to 90 cm (35") below the top of the coil did the trick: tuned close to the SSTV calling frequency (14230 kHz), SWR of 1.4, and a bandwidth of about 400 kHz between the SWR=2 frequencies. Just taped the bottom of the coil to the PVC mast. Obviously when the wind moves the coil, the SWR moves around a bit (1.2-1.5).
Antenna analyzer plots with free-hanging Slinky (left) and "tuned" Slinky
(right)
With 50 watts, I got an RSV of 575 from Harm, PD0CIF, in Holland (about 1200 km, 750 miles) and 595 from Giorgio, IW3SLQ, in the far northeast of Italy (about 840 km, 525 miles).
Last update:
15 July
2008
©2008 F. Dörenberg N4SPP
