Small Capacitive Antennas
Over recent years, I have tested several of the emerging small size capacitive-type antennas for HF bands. This does not mean that these antennas cannot be used on VHF or even on higher frequencies. The antennas tested are the Dual Disk, ”ISOTRON” type and Dual Wire antennas.
In all of the antennas I used the same and consistant approach to tuning: a single coil in series with the capacitive radiator of different shapes. The Dual Wire antenna shows that Ted Hart’s EH-antenna is operationally the same antenna as M.C. Hately’s Dual Loop Antenna. The ”L” and ”C” components have just changed place. This is solely my own opinion.
The DDA

The first of these antennas mentioned and tested is the Dual Disk Antenna, which is a design from a few year ago that I found when I first examined the antenneX web pages. I ignored it because the CFA tuner was over-shadowing everything else and more interesting at that time. A few months ago I found Ted Hart’s capacitive antenna pages on the web and I started to think of the feeding method used there also for the other capacitive-type antennas.
I decided to try this method with the Dual Disk Antenna, the antenna which was appreciated by so many hams, who had tested it. I made my DDA from the components which were used previously for my original CFA. I had to make only one 40-cm diameter disk. My DDA was 45-cm high with 40-cm diameter disks and the cylinder was 25 cm high with a 20-cm diameter aluminium tube. The isolators were 10 mm PVC bars with a length of 10 cm.
I connected the two disks together and to the feeding cable’s outer shield. The center wire was connected to a tuning/phasing coil and from the coil’s outer end to the cylinder.
The antenna resonated on the 40-meter band with a feeding impedance of about 10 ohms. I used a 4:1 transformer and the operating bandwidth was something like 70 to 80 kHz, so I tuned the antenna to 7060 kHz , which is the frequency where I can find Finnish amateurs most often. I used some ferrite beads over the cable near the feedpoint to prevent cable radiation. I had contacts to Lillehammer in Norway, Jallivaara in Sweden and Kemijarvi in northern Finland about 700 km from my station and the antenna was situated inside below a sheet metal roof.
Then I tuned the antenna to 80 m by adding inductance to the phasing coil. I succeeded in phasing the antenna and the feeding impedance had changed to near 50 ohms. The SWR was about 1.5 to 1 at the tuning frequency of 3700 kHz and with a frequency range of 200 kHz where the SWR was below 2 to 1. I used the antenna on 80 meters during one weekend and had contacts with distances from 0 to 600 km—again the antenna was placed inside in my house.
The ISOTRON

The ”ISOTRON” type design used and tested was made of two roasting pans, one above the other and the upper pan was upside down over the first one. The dimensions were about 30 x 40 x 11 cm so that there was a 1-cm gap between the pans. I used 11-cm long PVC bars to hold the pans together. The coaxial connector was screwed on to the lower pan like the tuning/phasing toroidal inductor (T300-2 material) with enough turns to tune the antenna (capacitance 35 pF) to 80 meters. The feeding impedance was again low.
Now I used a capacitor from the coax connector to ground and added two turns to the phasing coil. The capacitor must be of good quality because otherwise it will get warm and the tuning may sift a bit. The tuning range of this antenna was about 60 kHz at 3.7 MHz. I tested this antenna by talking to distances from 0 to 600 kilometers from my station, again with the antenna being inside in my house.
The Dual Wire

The Dual Wire Antenna is the latest design to emerge of these small capacitive antennas. It is a wire version of Ted Hart’s EH antenna. Its length is 8 meters (1/10 wavelength) on 80 meters band. The antenna is tuned with a serial coil between the coaxial cable’s inner wire and the other wire of the antenna.
The counterpoise is the other wire of a dual wire conductor (2 x 0.75 mm^2). The capacitance between the two wires, about 275 pF, reduces the phasing coil inductance. I resonated the antenna with 13 turns on a 32 mm diameter coil with a 5-cm length on a PVC pipe. The antenna’s feeding impedance is again low and I use the the same impedance matching technique as before. I have a capacitor of 1 to 2 nF of good quality across the antenna connector and I add 1 to 2 turns to the phasing coil. The operating range of this antenna is quite broad on 80 meters at about 180 kHz with less than 2 to 1 SWR ratio. I have had contacts from my station up to 600 km distancies and again the antenna being inside. My testing is an ongoing effort and this antenna seems to be a cheap and neat antenna for casual contacts from places where decent antennas are prohibited.

The operation of this antenna is quite clear when comparing the pictures of this antenna and the picture from my earlier article (antenneX February 2001 issue ”Dual Loop Antenna Designs” – article #64 Archive IV) where I described a shorted coaxial antenna derived from a Dual Loop Antenna. Only the L and C parts have changed places, otherwise the antennas are identical.
NOTE: The capacitor used in the Roasting Pan antenna can be fixed and does not need to be adjustable, except such is useful for the impedance matching. Tuning is done by changing the inductance which is facilited by having a variable inductor “L”. The same applies to the DDA.
Same Operation, but Different Field Flow
These two antennas may have separate patents because of different mechanical construction but the manner of operation is identical. RF currents are flowing to opposite directions in two nearby conductors making electric and magnetic fields of the same shape and phase allowing the Poynting Vector dynamics to occur. The other difference is how the fields are generated. In the DLA, the fields are generated around a bifilar inductance. In the EHA, the fields are generated across a capacitance. Again, this is solely my own opinion derived from my experiments with these capacitive antennas.
Originally posted on the AntennaX Online Magazine by Heikki Antman, OH2BGC
Last Updated : 28th May 2024