The beginning
An old friend from my Zodiac Sweden days asked me whether I knew anything about the EH Antenna. I hadn't the bleakest idea what he was on about. He gave me a link to the inventor and patent holder of this antenna. I read through the "EH Antenna Book" on Ted Hart's Home Page and I started searching for suitable material to try to build this peculiar thing Ted calls the EH Antenna. Surely a thing like this can not work! That was my first reaction. Seems to be anybody's reaction. Some "expert" in Sweden has referred to the EH Antenna as "a dummy load and an April fool" !
But... you really have to forget all you learned about Hertz antennas! It is actually more like a radiating capacitor!
An extremely compact CB antenna?
Although this "radiator" has been described mainly for the Ham bands, my aim was to get it on the air for my Packet Radio CB Station on 27 MHz. The reason for this is its compact size. Our local council do not like aerials cluttering up the landscape.
Collect the necessary bits and pieces
Now, the work had to start somewhere. Having studied Ted Hart's Home Page and read through his EH Antenna Book, a list of material was put together:
- PVC tube as support (do not use ABS)
- Copper wire for the coils
- Trimmer capacitors for phase adjustment
- Copper foil for the cylinders
- Coaxial socket for the antenna cable
- Glue to fix the coils
Choose the antenna pipe
The 27 MHz band falls roughly between the 10 m and 12 m amateur bands. Therefore I calculated a diameter for my CB band EH-Antenna based on data in a table found in Ted's Antenna Book.
According to Ted Hart's Standard EH Antenna Table, the diameter for an EH Antenna for the 10 Meter Amateur Band should be 1 " (25.4 mm). The middle of the CB band is roughly 11 meter, so 11 x 2,54 gives 28 mm diamter. As I could not find 28 mm pipe material, I decided on a piece of PVC electrical installation pipe with an outside diameter of 32 mm, which I found at the local electrician's. The make is PLICA KIR-K, part number 10.1104.532. It comes in 3 meter lengths and it costs only a few Swiss francs (or USD if you like).
Wire for the coils
The material for the coils is 1 mm diameter PVC insulated house installation wire. The trimmers for phasing were found in the junk box. They are simple 5-30 pF Philips air trimmers taken from an ancient TV tuner. They were later replaced with good quality, high Q, Philips trimmers with PFTE insulation. With a maximum of 4 Watts from the CB rig they should be sufficient.
The cylinders
The EH antenna consists of two cylinders. According to Ted's Antenna Book they should be 3.1416 x pipe diameter in length and should be spaced by one tube diameter. 32 x 3.1416 = 100 mm. The total antenna length adds up to 232 mm. Imagine an aerial which is 2.1 % of the full wave length! Compared to the Ground Plane antenna with its 2.7 meter radiator and the equally long radials it is really a small wonder! Or the lambda 5/8 Ground plane...
I chose copper foil for the cylinders as I had a roll of 19 mm wide Scotch copper foil adhesive tape lying around. Maybe not the cheapest choice, but it was available. Any sheet metal will do; for instance you could use kitchen aluminium foil and glue it to the tube. On the other hand copper is simple to solder the wires on to.
The matching network
The impedance of free space is 377 ohms. The radiation resistance of the EH-Antenna is 2 * 3.1416 * 377 = 2369 ohms. This impedance we need to transform to match our Transceiver's 50 Ohms impedance.
The coil capacitance can be calculated from (100 * SQR2 * 3.1416) / f. For 27,185 (center of the CB band) the capacitance is calculated to (100 * 1.414 * 3.1416) / 27.185 = 16.3 pF. This value is important for the function of the EH Antenna.
The coils can be roughly chosen from a table found on the W0KPH home page. The lower coil (L1) should be about 6 turns and the upper coil (L2) should have one or two turns more. My first EH-Antenna coils consists of 6 turns for L1 and 6 1/2 turns for L2. The phasing capacitors should according to tables be 16 pF for 10 meter; make that the same, 16 pF for 11 meter. It is a good start.
Checking the coils
To check whether the coils would work on the CB band, I close-wound 6 turns of wire on the PVC pipe and connected a capacitor consisting of a trimmer capacitor tuned to 16 pF in parallel to the coil. The grid-dip meter indicated a dip at about 27 MHz. Based on this result I added one turn for L2, mounted the coils and the trimmers on the PVC pipe and connected the cylinders and the SO239 to the phasing network. Remember, we are not tuning any coils to a certain wanted frequency, we just want to phase out the reactance in the EH antenna. If you want the full story, please read Ted's Antenna Book.
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AN EH-ANTENNA FOR 27 MHz
The cylinders in place. The UM-3 battery for comparison is 50 mm long. Compare the size of the EH Antenna to the computer keyboard!
Close-up of the construction. Phasing the EH Antenna in the shack. Each cylinder is 10 cm long.
On Air for the first time. Or at least "in the air" for the first time. The trees are the neighbour's and are more than 20 meters away.
A useful Grid-Dip Meter for checking the coils. This gadget was handy to keep a track of the EH Antenna construction.
Impedance bridge with a built-in crystal oscillator for 27.255 MHz. Handy device to determine a multiple of a half wave length coaxial cable for the operating frequency.
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