Building the 14mhz and 18 mhz EH antenna’s

I had problems building the 18 Mhz antenna using a phasing network as in the other article due to the small size of the coils. As my grandmother used to say there is more than one way of skinning a cat and the second method is usually simpler so I decided to give the pole antenna design a go of Jack W0KPH(http://www.qsl.net/w0kph/). If you think about it we want to put AC across the fat dipole capacitor 180 degrees out of phase on each capacitor plate so using a transformer is another natural way of doing it. The primary of the transformer is where the RF goes in from the transmitter and the secondary goes to the two capacitor-fat dipole plates.

We are attempting a resonant circuit at the frequency we want to transmit at plus an impedance transformation of approximately 8000 ohms down to 50 ohms to suit the transmitter.

The capacitive part of the tuned secondary is provided by the natural capacitance of the fat dipole. The inductance is the transformer secondary coil plus some mutual inductance across the transformer to the primary winding. In practice in the design shown below I used 3 inch diameter aluminium tube of 18 swg wall thickness its length being PI multiplied by 3 inches long which is 9.42 inches for each tube in the dipole. 68 mm water pipe was used as the mounting former and as before this was mounted inside a 4 inch water pipe as the outside rain cover. The two cylinders were set at one end of the 68 mm pipe spaced by 3 inches. In my case I saw no reason as not to wind the transformer in the space between the cylinders. To do this I wound half the secondary turns up to a 5/16 inch hole that was drilled half way between the cylinders. The turns were widened to clear the hole and the rest the same again onto the solder point on the other cylinder. The solder points on the cylinders were solder tags fixed on with 4ba screws. I then fed the end of a piece of 50 ohm UR43 co-ax through the hole stripping approx ½ inch at the ends so it was just through the hole. To this I fixed one turn of 18 swg wire connected around the tube loosely between the co-ax braid and its center conductor to form the primary winding. There were 14 turns of extra flex wire for the 18 mhz antenna and 16 turns for the 14 mhz antenna. Remember this is an EH antenna as you raise it in height from ground level the resonant frequency goes up so whatever you set at 4 feet will not work at 12 feet etc etc. As an example my MFJ antenna bridge showed resonance and match at 4 feet of less than 13 mhz while at the working height of 14 feet to the dipole center it was correct on 14.200 mhz. Similar things apply to the 18 mhz antenna. I am sure there must be a graph that would show frequency against height but you would need a hi-lo mast to do the measurements.The method is to first over wind the secondary , put the antenna to the final height and measure it there. Remove turns until resonance is achieved. This version of the antenna is very easy to make and set up compared to the other version using capacitors and inductors in a phasing network. All of the tests and setup were done with the outer rain cover in place. The co-ax cable next to the socket on the bottom of the rain cover had ferrite sleeves put over it. Originally when experimenting I ran both antenna’s at 4 feet above the ground, working all across Europe on 14 mhz and getting reasonable reports. Tests with my sniffing rf field strength meter showed no field at the bottom of the antenna from 4 ft out from it but as I raised the sniffer the RF levels increased , giving the impression of a radiation lobe 30 degrees or so from the horizontal. There was still some RF on the short length of coax feeder, the majority of feeder was on the ground and at RF earth. From the previous experience of making these antenna’s it took me 2 hours to make each of these including set up time to working the first stations. Using the pipe shown as a mast I can swap from antenna to antenna in 3 minutes flat and I can store them away with the mast in the same time between usage in the garden shed. The pipe at £12 for 2.5 metres is the cheapest radio mast I have bought yet. If you want a permanent strong mast then set it in the ground and fill it with concrete.

For the circuits to this antenna go to Jacks Site W0kph at the address shown above as it is his design. This example is how someone in the UK can realize this type of antenna and my method of making it.

Making the Core

This picture shows the smaller inner plastic drain pipe with the windings. The short link connects the co-ax braid to the top of the bottom cylinder.

Mounting the core

This is a picture of the end cap showing the plastic plug glued to the end that centers the one pipe inside of the other.

The other end is fixed in place with a metal spacer and a 2 ba bolt. There are probably other things that can be used

As the end locator plug but they must be glued on.

Loading the munitions

At least that’s what it looks like as you feed the core up the rain cover.

And Finally connect up the cable to the socket on the bottom of the pipe

Note the ring cores on the co-ax and the spacer. This spacer can also be

Made out of a bolt with nuts as the spacer.