A Multifunction Antenna Support
A few days ago I was wandering through one of the local large sporting goods stores, looking for anything that I might find useful for antenna projects. I spotted a large stack of fiberglass collapsible fishing rods. Several were 16.5 feet long (5.02m) when extended. Collapsed length was 4 feet (1.2m). This length turns out to be right at 1/4 wavelength on 20 meters. My idea light bulb went on. I bought one and after a lot of thought, the following antenna ideas materialized:
These antennas are for the QRP crowd and backpack hams that go into the wilderness and operate. It is not that the 100-watt transmitters can’t be used, but for the most part, the QRP hams will like the portability of this antenna system. Also if the bands are not any good, at least you can go fishing. Sometimes I think that is better than fighting the QRM on the ham bands. And it is relaxing most of the time, depending on what you catch. Saltwater fishing is rather unpredictable, as you never know what will be hauled in, sort of like DX. Sometimes, stingrays, sometimes redfish, you never know. Same thing with DX.
The first idea was of a 1/4-wave vertical with a matching and loading network and radials at the base. If the antenna wire is gently spiraled around the pole it will not flop in the wind. About two turns along the entire length will be more than enough. A matching and loading network is a good idea to allow you to tune out any variations that may occur at different locations. This antenna is easy to implement. Plus, using 1/4-wavelength wires, cut for the 18, 21, 24, and 28 MHz bands, used as needed would give multi-band capabilities. Figure 1 illustrates this type of 1/4-wave vertical.
Then it occurred to make a trap vertical antenna which is shown in Figure 2. You could use small variable capacitors to tune the coils to resonance instead of tapping a coil and using a fixed capacitor. Either way, this is for power levels below 10 watts only—NO MORE—as the resultant voltages will cause the mica capacitors to arc and destroy them. A grid dipper can be used to resonate the traps when they are in place. The traps should be inserted at the top of each 1/4-wave section. In other words, the 28 MHz trap is to be inserted at the top of the 1/4 section. Each trap that follows is to be inserted at the top of the corresponding 1/4-wave section for that band. Again, a matching network should be used to take care of any irregularities that may occur at different locations.
By adding a top hat, the antenna can be used on lower bands as long as you have a good ground /radial system. The use of a matching network is required of course. Keep in mind the antenna will be only 16 feet high and this will cause it to have a high angle of radiation on the lower frequencies. This can limit the DX serious capabilities of the antenna under most conditions. The top hat is illustrated in Figure 3. A matching and tuning circuit is needed with this antenna as well as a good ground and/or radials.
Another antenna you can make from this idea is the quarter-up and quarter-out antenna. This is a folded 1/2-wave antenna and is a fairly decent antenna for DX. This antenna needs a ground and antenna tuner to match it to the transmission line. It is also fairly simple, as are most of the others described.
| Now a look at how to resonate a coil and capacitor without a grid dip meter. Figure 5 illustrates the method used to resonate the coil and capacitor. How it works is like this. If you remember from basic tuned circuit theory, a parallel tuned circuit represents high impedance at resonance and a series tuned circuit, low impedance. Therefore at resonance if you have connected the parallel circuit in the antenna as shown in Figure 5, any signal at the resonant frequency will not get through the parallel circuit. The receiver will hear a decreasing noise and signal level as you tune towards the resonant frequency of the parallel tuned circuit. After you pass the point of least noise and signals, the noise and signal level will build up. This will give you a very good idea of where your circuit will resonate and you can make the required adjustments if needed. |
Hope you enjoyed these ideas as much as I did! More advances uses of these ideas will appear in future issues.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 17th March 2024