T300 – VHF Antennas for Home/Mobile

This module covers the basic types of VHF (and UHF) Antennas that you are likely to encounter, the advantages and disadvantages of each, and how to select and/or build a VHF Antenna for your station.

VHF Antennas for Portable Radios

A note about most VHF/UHF FM signals…

There are two things that you probably studied to get your license but probably didn’t understand very well: Polarization and Modulation. Both are very important to understand how most communication works on Ham Radio VHF/UHF bands.

Most signals on ham VHF/UHF bands are Vertically Polarized, meaning the main radiating element of the antenna is vertically oriented. If there is a mismatch between the polarization of radiating elements, there is a built-in loss between the two antennas, so most people choose to use vertically polarized antennas.

Most modulation on ham VHF/UHF bands is FM (Frequency Modulation). A basic tenet of FM is the “capture effect” which basically means that the strongest signal wins and all others lose, so it’s very important to have the strongest signal possible.

The Basic “Rubber Duck”

[rubber duck image ] Most HT-type radios come with a “rubber duck” type vertical antenna that is connected to the top of the radio. It’s called a rubber duck because it’s flexible and bends easily. Electrically, the antenna is usually an electrical quarter wavelength of wire, but helically wound to make the physical length much shorter.

The basic rubber duck antenna is a (poor) compromise between size and electrical performance. It will usually work for very short distances, but should not be counted on for communication over several miles.

Since repeaters are usually located on high locations and have considerable power, repeater signals can be received by HT rubber duck antennas without much problem. But HTs trying to transmit to repeaters miles away through a rubber duck usually will have poor performance – scratchy at best. New hams wrongly assume that, just because the received signal is clear, that their transmit signal will also be clear – that’s not usually the case.

A Full Quarter Wave Whip Antenna

[signal stick] If you need solid performance for more than a couple of miles, you should probably upgrade your HT antenna (or consider using a mobile radio). If you are portable, you’ll still want a basic vertical antenna attached to the radio, but there are better antennas to choose from. They are longer and slightly less convenient, but the performance is usually worth it.

A 1/4 wavelength at 2M repeater frequency (146 MHz) is roughly 19 inches long, so a fully straight 1/4 wave vertical is going to be 19 inches long, but will provide the best performance available for that type of antenna. A Signal Stick antenna is a very good alternative to the rubber duck. Another larger antenna is a Diamond/Nagoya 17 inch flexible antenna. This antenna is base loaded but the main part of the antenna is much thinner since it’s just a flexible wire encapsulated in a protective cover.

Note that when you are at home or mobile, you can also improve your signal by using external antennas (see below) with your HT.

VHF Antennas for Mobile Radios

Quarter Wave Magnet Mount

The most popular auto antenna for UHF/VHF is probably the quarter-wave magnetic mount. This very simple antenna works well for most all types of vehicles (at least the ones that are made of steel). We are lucky as hams in that 1/4 wavelength at 2 meters is also approximately 3/4 wavelength at 70 cm (our 420-450 MHz UHF band), so a common antenna will work well for both. Since the vehicle body is half of the antenna, placement matters. The optimum location is the center of the roof (so that the ground plane is roughly equal in all directions). If that is not possible, the center of the trunk is the next best placement. Occasionally, the antenna will need to be mounted to either the very front or very back of the vehicle – doing so will result in a directional signal.

Vehicle antennas are often compromises between what is possible to mount and where mounting is possible. Do the best you can, but higher and more central is better.

Longer Wavelength Antennas

If you have to mount to the bumper or adjacent location (especially with a truck or SUV), you can recover some of the performance by using a longer antenna, which will have some small gain over a traditional quarter-wave vertical. You can use the longer length to try and extend above the roofline so that your signal goes in all directions.

VHF Antennas for Home Stations

There are more antennas that are popular for your home VHF Station – some are similar to mobile antennas, but others are larger and more complicated because the space available is larger than what is available on a typical vehicle.

Quarter Wave Ground Plane

As with a vehicle, the most simple antenna is a quarter-wave vertical. They are cheap and easy to build (see right) if you want to try it yourself. If not, there are several good quality pre-made antennas for purchase. As with a vehicle installation, the highest mounting possible will get you the best performance. Since a house mounting is usually not to a flat plate of steel, roof-mounted antennas usually need some form of artificial ground plane (in the form of horizontal radials). It’s also important to use a high-quality feedline and keep the cable length as short as possible to reduce signal loss (see right).

Longer Wavelength Vertical

Also as with a vehicle, you can always use a longer (multi-wavelength) antenna to improve your signal. Many commercial antennas exist for VHF/UHF that are 6-12 feet long, and these have significant gain and a lower radiation angle than a single quarter-wave vertical.

J-Pole

A good easy-to-build antenna that has a low radiation angle and is fairly simple to mount is a “J-pole”. It’s called that because the antenna resembles the letter J. Basically, the J-pole is a half-wave radiator and a impedance matching stub. Aside from being easy to build, it’s a good antenna for two reasons: It has a lower radiation angle (compared to a 1/4 wave vertical) and it’s a “DC ground” antenna (which means it will bleed off static and lightning-induced charges). There are hundreds of J-pole variant designs found online, made out of a variety of materials, but the two most popular are j-poles made out of small-diameter copper tubing and those made out of “twin-lead” balanced feedline.

Yagis and Other Directional Antennas

If you need gain and/or directionality (for example to reach a far-off repeater or an orbiting satellite (see T700), then you will need a “Yagi” or some other directional antenna design. The Yagi-Uda design has been around for many years and is very popular because it works well and is fairly simple and requires a minimal amount of space. The traditional Yagi consists of parallel antenna elements mounted on a “boom” (horizontal support). The magic in a Yagi is the dimensions of the elements and the inter-element spacing. You can read about the theory or you can just buy or build one and use it.

Build or Buy?

Many new Hams purchase premade antennas.

Your First Home 2M Antenna

Build a simple 1/4 wave vertical!

Feedline Matters!

The feedline is the cable that carries the transmitted energy from the radio to the antenna. There is usually no feedline for an HT radio since the antenna is directly connected, but the type and length of feedline matters greatly for mobile and home installations.

Feedline has losses and the amount of loss varies with both the type and length of feedline. It is very important to use a feedline that has low loss for the frequencies that you anticipate using. Some coaxial, or “coax”, cables that work fine for HF signals are way too lossy at VHF/UHF. If you don’t know which type to choose for your particular installation, ask your radio dealer or a more experienced ham.

Callum M0MCX has a short piece on the selection of coax. As Callum says, you need to know 1) What is the maximum frequency you are going to use, and 2) How much loss are you prepared to accept? From that, you can select the coax that you need.

Callum M0MCX discusses coax selection.

A Note About Connectors…

As with feedline, the type of connectors matter. Most mobile radios at VHF use the traditional “UHF” (PL-259/SO-239) connector set mainly because they are cheap. HTs use a variety of SMA connectors for the same reason. Anything higher than VHF should use a connector designed for 50 ohm characteristic impedance at RF, so it “matches” the impedance of the feedline. Both Type “N” and Type “BNC” meet this requirement and will be found on better and higher-power equipment.

It’s also a good idea to standardize on a connector type (either through cables or just adapters) so that you can hook any radio to any compatible antenna.

Antenna Polarization

As I mentioned earlier, you need to know the polarization of your antenna, because a mismatch between the source and destination antenna polarization can really reduce signal strength.

Most VHF/UHF repeaters are vertically polarized so your antenna also needs to be vertically polarized.

VHF/UHF “weak signal” work (see T800) is mostly horizontally polarized so you will need a horizontally polarized antenna. If you try to use your “repeater” vertical to contact a horizontally polarized weak signal station, you might be calling for a long time with no response.

Satellites are a special case as satellites are constantly moving and they tend to spin in orbit. This means that sometimes your antenna will match and sometimes it won’t, resulting in deep fades on the satellite signal. Most people that communicate through satellites use antennas that are Circularly Polarized – a special combination of horizontal and vertical elements that cancels out the fade that results from satellite spin.

Next: T400 – Digital Voice Modes & Inter-Networking