Linear Antennas 

Building “linear” (short) antennas to fit restricted space requirements

Co-Linear loading is folding apportion of the antenna back against itself, in order to reduce the physical length of an antenna to the restrictions imposed on the design. This technique has been used for decades in dipoles antennas; linear dipole and double bazooka dipole are examples. Generally the physical length can be 60% to 70% of a conventional quarter wavelength size without RF performance changing. As an example case; consider a 22 Foot Fiberglass hollow flagpole you want to use as a 40M vertical antenna.

The problem with flagpole antennas or any short antenna matching. A poor match is a killer for losses. As you are using a remote tuner you have the match covered for most cases. It would be an advantage to use the linear antenna inside you’re pipe for your flagpole as it will help tuning on 160M – 40M. The linear antenna is not better or worse for radiation, it just makes the antenna look electrically longer. Each results in a compromise between the band, feed length and the location. You should carefully consider your tuner performance to see the predicted capacitance and inductance is exceeded with margin before making a decision Remember an “Antenna System” is the radiating element(s), matching network (tuner) and feed working together.

Short Antenna Remotely Switched Load Coil

The first step is to decide which bands and frequencies are desired for the tuning system. In most cases the 2:1 SWR bandwidth will be the entire band for 40M, 30M, 20M. The coil approach will not work above 14 MHz (20 M) as the antenna becomes too long and a capacitance is required instead of inductance. The 80M 2:1 SWR bandwidth will be 150-180KHz so plan on 1-3 depending on the mode(s) you want to run. This means the design could require 5 relays for 40-15M and 3 for 80M for a total of 8 relays. Considering tune the vertical for 40M to work okay as a 3/4-waveIength vertical on 15M and the same for 30M. In the real world 40M and 15M are the same tap, 80M and 30M are the same tap if you only operate in CW mode in 80M requiring 1 tap for a total of 6 taps (or 6 relays). Now six relays is a more manageable design. Note: Relays should have a 600 VAC contact rating for 100 Watts RF. 

I recommend you make your coil from bare #10 AWG Copper wire winding the coil about 40 turns tightly without spacing around a 3 to 4 Inch form like a pipe or can. A bigger diameter is better as the capacitance will increase in a smaller coil. The wire will be stiff so you will need a helper. Back off the tension to remove your form and stretch the coil out so you have 4 to 6 turns per Inch spacing. Now you can add terminal lugs to mount your coil on a sheet of acrylic plastic sheet with the relays. When selecting an enclosure for your remote load coil be sure the box is big enough to keep the coil 1 ½ Inches from plastic and three Inches from metal. Note if you are using a metal box the VSWR will change when opening and closing the cover of the metal box. Plastic outdoor electrical boxes are recommended.

The dimensions and coil settings given above should produce reasonably low VSWR readings over the entire 15, 20 and 30 meter bands and over at least 250 kHz of the 40 Meter band. Bandwidth on 80/75 meters should be at least 100 KHz for VSWR of 2:1 or less at the low end of the band and may be as much as 200 KHz at the high end of the band, depending on the efficiency of the ground system used, greater bandwidth being associated with poor ground systems. 

Once you have built a 17 Foot flagpole antenna and the yard is filled with 64 buried radial wires to make a counterpoise you are ready to make your own matching system. Now you are ready to start measuring to set your taps the first measurement should be finding the basic resonance of the antenna. Note VSWR is not best VSWR, resonance is when the antenna is purely resistive, 14 MHz depending on the element RF length (16 Feet) selected. VSWR is a ratio to your 50 Ohm cable. Also low VSWR by itself does not mean that a vertical antenna is operating efficiently. If a low wide bandwidth VSWR is obtained likely means the opposite. This condition means you will have improper tuning of antenna can usually be attributed to inadequate (or even reactive) ground systems or interaction with objects in the vicinity of the antenna. For these reasons it is suggested that the antenna be placed as much in the clear area as possible and used with the best ground system that conditions permit.

It is reasonable to expect your best results on 20M with a VSWR at the base of the antenna less than 2:1 across the band. For taps you should start at 3900 KHz to find the 80-meter relay lead was touched on the turns of the coil until minimum reflected power was indicated. Solder the wire in place on the coil only after all bands are tuned. Next try 7150 KHz and touched the 40-meter relay lead to the coil turns until an SWR of 1:3 is read. While using the same coil tap measure on 21.1 MHz and checked the SWR. If the VSWR is less than 2:1 you are done, but most will need to move the coil tap just ½ to 1 turn you are able to get an SWR of 1.8:1 on 15 meters. Now recheck on 40M is required and this process is repeated until both 40M and 15M Bands are less than 2:1. The same process is required for 80M and 10M. After that you can move on to 20M. Note if you are using a metal box the VSWR will change when opening and closing the cover of the metal box. Plastic outdoor electrical boxes are recommended. Now hook up your coaxial cable and move to your shack to check each band again. Remember the length of the coaxial cable can cause VSWR mis-match. The 40M ¼ wavelength antenna is dependent on the coaxial feed length. The general advice is use cable lengths of 40-50, 70-80, 100-110 or 130-140 feet and DO NOT use cable lengths of 30, 60, 90, 120 feet.