Frequently Asked Questions - Antennas

Why should I use a Balun?

          Baluns are used to convert your unbalanced coax cable to a balanced signal to feed antennas with a balanced input like dipoles.  If you do not use a Balun the unbalanced feed line could skew the pattern of your antenna.  Also there could be RF on the outside of the coax which could come back down the coax and into your shack.

What kind of Balun do you sell?

          Our Balun is a 1:1 current Balun as designed by Jerry Sevick, W2FMI.  One to one Baluns are used to convert the 50 ohms unbalanced coax to a balance 50 ohm feed to match a 50 ohm antenna.

What is the impedance of your half wave center fed dipole antennas?

          In free space the impedance is 73 ohms and is slightly inductive.  Unfortunately in the real world the answer to this question is: “It depends.”  The impedance varies with the height above ground, the thickness of the wire, the nearness of any metal structures, etc.  Our 20 meter antennas, with Balun, out in the clear at 20 feet above ground give us SWRs of about 1.7:1 and a 3:1 SWR that is much wider than the 20 meter band.

How much power can your Balun handle?

          Jerry Sevick, W2FMI, rated the design that we are using to handle the full legal limit as long as the SWR of the antenna is below 2:1.

When I hang the antenna how much tension should I use?

          The antenna parts were tested to hold up to 100 pounds of pull.  The #14 hard drawn 7/22 wire that we use is rated to withstand 200 pounds of pull.  The ARRL Antenna Handbooks recommend that you not put more than 10% of the tension on the antenna that the antenna wire can handle so they are recommending a maximum of 20 pounds in this case.

What kind of hardware do you use in your antennas?

          All of the hardware used in our antennas is 18-8 stainless steel.  The designation 18-8 means that the stainless steel has about 18% chromium and about 8% nickel.  18-8 stainless steel is slightly magnetic.

Why did you use hard drawn wire?

          Hard drawn wire is already pre-stretched by drawing it.  This means that the wire is less likely to stretch when you hang it.  Soft drawn wires have not been pre-stretched and so your antenna built with that kind of wire could lengthen over time and become non resonant.

How do you install half wave center fed dipoles?

          There are two main ways to install the half-wave antenna: as a straight dipole and as an inverted V. In either case the antenna will work better the higher you can put it up. As a straight dipole you tie ropes to both end of the dipole and raise it as high as is possible. If what you are tying both of your ropes to are both fixed objects like buildings or towers you should have no problems. If, like most people, you are using trees then you have to allow for the trees to move. It is better to only tie one end securely and to use a spring or pulley and weight system to hold the other end. Most of the Antenna Handbooks that are available will go into detail about how to do this.

          To install the half-wave dipole as an inverted V you need three tie points but only one of them needs to be up high. Using a rope tied to the top-center eye bolt on the Dipole Center Insulator pull the center of the antenna up as high as possible. Tie off the two ends to supports that are lower down. Again, if the supports can move, use springs or a pulley system to reduce the strain on the wires.

          There are several alternate ways that people have installed half-wave dipoles and have had some success. Some people have installed them in their attic and stretched them from end to end. Others, who did not have much room, have installed them with twists and turns in various places. Be sure to use insulators and any point where the antenna wires can touch. If you do something like this you will have to carefully tune the antenna.

How do you tune the antennas that are adjustable?

          The Half-Wave Dipole Kit antenna will need to have its length adjusted to tune the antenna to the frequency that you want to use it on. On the higher frequencies you should be able to tune the antenna to the center of the band and it will then work over the whole band. The 80 Meter band is so wide that you will have to pick a portion of the band over which you want to operate and tune for the center of that portion.

          Tuning the antenna is done with some kind of meter that will measure reflected power. SWR bridges or watt meters are commonly used. Most transceivers now have a SWR bridge built in and that will work fine. You need to tune the antenna because of many factors like how high it is installed, what kind of ground is below it, what kind of buildings or structures are nearby, etc.

          To tune the antenna first refer to the chart below and pre-cut both ends of the antenna to be about 12 inches longer than the lengths given for the frequency you want to use. Both sides of the antenna should be the exact same length. Then install the two end insulators at the end of the wires using the extra 12 inches. Do not solder or permanently attached the ends at this time.

          Next install the antenna as close as possible at the point that you will be leaving it permanently. Check the SWR at intervals over the whole band and record the readings. Most likely the antenna will be too long which means that the antenna will be tuned below the frequency at which you want to use it. This will be shown by the SWR increasing as you move up in frequency across the band. If this is true shorten both ends of the antenna in stages until the lowest SWR point is at the center frequency of the band or the frequency you will use it at the most. At that time securely wrap the wire at the ends by the End Insulators and you can then solder the two wires together or tie them off with cable ties.

          The chart below is based on the number that most people use for calculating how long a half-wave antenna should be for a given frequency. That number is 468 divided by the frequency in MHz.  That will give you a length in feet for the overall antenna length. Each side of the antenna should be the same length so each side will be one-half of that length. For instance if you want to tune your antenna to 14.25 MHz then 468/14.25=32.84 feet or 32 foot 10 inches. Each wire on the antenna should measure one-half of that number or 16 feet 5 inches long from the center of the Center Insulator to the end of the wire in the End Insulator. That is the length you should start with to tune your antenna. Don’t forget to add 12 inches to the end of the wire to have some to wrap around the end insulator.

80M  3.55 MHz 131 Ft 10 in 65 Ft 11 in
80M  3.7 MHz 126 Ft 6 in 63 Ft 3 in
80M  3.85 MHz 121 Ft 7 in 60 Ft 9 in
40M  7.05 MHz 66 Ft 5 in 33 Ft 2 in
40M  7.2 MHz 65 Ft 0 in 32 Ft 6 in
30M  10.125 MHz 46 Ft 3 in 23 Ft 1 in
20M  14.075 MHz 33 Ft 3 in 16 Ft 8 in
20M  14.175 MHz 33 Ft 0 in 16 Ft 6 in
20M  14.25 MHz 32 Ft 10 in 16 Ft 5 in
17M  18.118 MHz 25 Ft 10 in 12 Ft 11 in
15M  21.1 MHz 22 Ft 2 in 11 Ft 1 in
15M  21.275 MHz 22 Ft 0 in 11 Ft 0 in
15M  21.375 MHz 21 Ft 11 in 10 Ft 11 in
12M  24.94 MHz 18 Ft 9 in 9 Ft 5 in
10M  28.3 MHz 16 Ft 6 in 8 Ft 3 in
10M  29 MHz 16 Ft 2 in 8 Ft 1 in
10M  29.4 MHz 16 Ft 0 in 8 Ft 0 in

Our present product offering:

Half-Wave Center-Fed Dipole Antennas (without Balun)

Half-Wave Center-Fed Dipole Antennas with 1:1 Current Balun

Antenna Center Isolator (without Balun)

Antenna Center Isolator with 1:1 Current Balun

Frequency Devices, Inc.
Your engineering partner for signal conditioning
  1784 Chessie Lane
Ottawa, IL 61350

Fax: (815) 434-8176


Phone: (815) 434-7800
or (800) 252-7074 (U.S. only)

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