General VHF/UHF Antenna Considerations

A high-level summary is presented here in quick, bulleted form.  More details are provided below. It is important to understand that the recommendations for antennas really apply to any simplex FM station, whether it is used for voice or data communications.

• Mount as high as possible and above local rooftops
• Use an omni-directional antenna
• Maintain vertical and horizontal separation, especially between antennas in the same band
• Fixed site antennas
  • A tri-band (144/220/440) ground plane antenna allows use on all currently used voice and packet bands, and most provide excellent gain
  • 1/2" hardline (such as Heliax) or low-loss, double-shielded coax, such as LMR-400 or LMR-600, is recommended for the feedline
• Field site antennas
  • A roll-up J-pole makes an excellent field antenna
  • A 32' telescoping fiberglass mast is highly portable and gets the antenna up above all one-story and most two-story buildings
  • Support the mast by strapping it to a tent leg or a sturdy tripod
  • If a tripod is used with a lightweight mast and a roll-up j-pole antenna, the tripod can usually be secured with sandbags and/or stakes, avoiding the need for guy ropes. Be sure to mark the area with safety cones.
  • If a tripod is used with a heavier mast and antenna, guy ropes will usually be required to support the antenna at 20+ feet high. Extra safety markers will be needed for the guy ropes.
• Use the lowest loss coax cable that fits your budget and is appropriate for your installation.
• Use an appropriate lightening arrestor

Antenna Height

• The height of the antenna above ground and other obstructions is the most important attribute of the antenna system and probably the most important factor of the whole station.
• The goal is for the station to hear and be heard by all other users on the frequency, thus avoiding the "hidden node problem" associated with simplex operations..
• VHF/UHF frequencies don't require strict line-of-sight between endpoints. But the fewer obstructions in the path, the stronger and cleaner the signal will be.
• Raise the antenna up as high as possible. At a minimum, the antenna should be above the rooftops of local buildings.
• 30 feet of height is easy to achieve with commercially available, lightweight, collapsible, fiberglass masts.
• A taller mast is better than a shorter mast. But a shorter mast is better than no mast. 30 feet is better than 20 feet. 20 feet is better than 10 feet. 10 feet is better than 5 feet. Do the best you can.

Antenna Pattern  (Omni vs. Directional)

• An omni-directional antenna is almost always the best choice.  This ensures that you have the best chance to hear all other users on the frequency and for them to hear you during simplex operation.
• Directional antennas can be useful in some specific situations, such as when you are far from a particular repeater.
• But directional antennas are usually not the best choice because they increase the likelihood of the hidden node problem in the nulls of the antenna pattern.
  • Remember that our amateur radio stations should be designed to deal with a variety of disaster situations. For example, maybe your usual repeater is down and you need to operate simplex, talking to a net control in a different direction than your primary repeater. Or maybe you are unable to reach your assigned primary BBS or e-mail server, requiring you to connect to a different node in a different direction. Directional antennas can make either of these situations difficult.
  • Another problem with directional antennas is location flexibility. Not all situations require operators in the same place each time. Some operators may be asked to relocate to another location, perhaps even another city. So, an antenna carefully selected for one particular location will most likely be a terrible choice for another location.
  • Another problem with directional antennas is portability. Any type of beam antenna is harder to store, transport and deploy in a field environment.
  • A direction antenna typically requires a heavier mast, which requires more substantial mast support, such as guy ropes. And guy ropes need to be marked so people don't walk into them or trip over them.
• Lastly, simplex operations, whether voice or data, are subject to the hidden node problem. Directional antennas dramatically increase the likelihood of creating a hidden node.
• Bottom line, for best results, an omni-directional antenna is almost always the best choice.

Antenna Gain

• Gain is far less important than antenna height.
• If mounted high enough, and in a good location, even a discone (zero gain) is sufficient to hear and be heard by the other users on the frequency.
• For most field locations within Santa Clara County, a J-pole (essentially, a vertical dipole) is sufficient.
• For most EOCs and other fixed locations, a tri-band (144/220/440) co-linear, ground-plane antenna is the best choice. Most co-linear, ground-plane antennas have plenty of gain.
• Most city EOCs use the 220 band for packet so they can use the 144 and 440 bands on the same antenna for voice operations.

Desense

• A strong, nearby signal will make it harder to hear the desired signal because the receiver is overwhelmed by the interfering signal, causing its sensitivity to drop.
• Desense can be mitigated by:
  • Moving the receive antenna as far away as possible from the noise source. In a multi-antenna location, separate the antennas vertically and horizontally.
  • Filtering the transmitter. There are licensing and certification limits to the level of spurious signals a transmitter can generate. But even a conforming transmitter with an antenna near to the receiver's antenna can produce enough spurious noise to desense the receiver. Filters on the output of the transmitter will reduce spurious signals that may be in the same frequency range that you're trying to receive.
  • Filtering the receiver. Receivers are most sensitive to the frequency to which they are tuned. But strong signals on nearby frequencies in the same band, or very strong signals in another band, can still desense the receiver. Filters on the receiver can reduce spurious signals that are outside the frequency range that you're trying to receive, but still strong enough to desense your receiver.

HT Antennas

• The "rubber duck" antennas on hand-held radios great for portable operations (walking, bicycling, etc.). They are sufficient to reach high-level repeaters and nearby simplex users.
• But "rubber duck" antennas are usually not good enough to be heard by all users on a city-wide simplex voice net. And they are terrible antennas for data networking since the other users on the frequency are distributed over several nearby cities.
• When possible, such as when at a stationary site, connect your HT to a higher gain antenna located up high. Many schools, shelters, fire stations and other locations have been equipped with permanently mounted antennas for this purpose.
• If an HT antenna is the only antenna available, then it's better than nothing. Just remember that using an HT antenna for simplex operations will cause the "hidden node problem", which will slow down the channel for everyone.

Feedline

• High quality, 50 ohm coax feedline is important for reducing signal loss. This is especially important for simplex operations and operations at UHF frequencies.
• Consult our General Coax Feedline Considerations page for more details.

Lighting Protection

• Beyond the obvious fire hazard that can be cause by a direct lightening strike, sensitive components in radio equipment must be protected from both the sudden surges caused by nearby lightening and the gradual accumulation of static charge that occurs due to wind and rain.
• Consult our General VHF/UHF Lighting Arrestor Considerations page for more details.

Notes on Some Specific Antenna Configurations

• Antennas
  • NCG Comet CX-333
    • The SCCo BBS sites, several city EOCs, Hospitals, and other fixed sites use the Comet CX-333 tri-band, co-linear, ground plane antenna. It is advertised has having 6.5/7.8/9.0 dBi gain at 146/223/446 MHz, respectively. It is approximately 10 feet tall, which makes it too large and heavy for most field setups. It is shipped as two sections which are joined in the middle during assembly. Although not mentioned in the assembly instructions, properly taping the joint in the middle will prevent water intrusion in the event that the o-ring fails. This is particularly important where the antenna may need to endure freezing temperatures and/or lots of flexing due to high winds, such as at mountain-top locations.
  • Austin Suburban
    • The SCCo EOC and some cities use the "Suburban" tri-band vertical dipole from Austin Antenna. It is typically used at a fixed site but, with a substantial mast, could be used at a field site. As a center-fed, half-wave dipole on all three bands (2.14 dBi gain), this antenna does not have the gain of the Comet CX-333. But the radiating portion of the Suburban is only about 4 feet tall, so it will fit on crowded towers where taller antennas will not work.
  • Roll-up J-Poles
    • Many field stations use roll-up j-poles from Ed Fong, a local RF engineer in Sunnyvale (http://edsantennas.weebly.com). J-poles are essentially dipole antennas, with the same gain as a dipole (2.14 dBi gain). The roll-up j-poles are available in dual-band 144/440 MHz (model DBJ-2) and single band 220 Mhz (model DBJ-220R) configurations. Because they are very lightweight, they can be supported on a tall but thin telescoping mast without making it too top-heavy. PVC-encased versions are also available for fixed locations, including a tri-band PVC-encased version, but a vertical ground plain or dipole is better. They are low cost, well constructed and tuned. MFJ also makes roll-up j-poles for several bands.
• Telescoping Masts
  • Many field stations use the 32' telescoping fiberglass mast from The Mast Company (http://www.tmastco.com/page_products_telescopic_poles.html). It collapses to 44 inches in length and weighs only 5 lbs. It is very thin at the top but can easily support a roll-up j-pole, putting the antenna above the height of all single story buildings and most two-story buildings. MFJ also makes a similar mast product.
  • A more substantial mast from Max-Gain Systems (http://www.mgs4u.com/fiberglass-push-up-mast.htm#telescoping) can be created. These masts are also portable, although they are heavier and take up more space. But they are capable of supporting heavier antennas, such as an Austin Suburban, regular j-poles, or ground plane antennas. Remember, antenna height is the most important factor. Supporting a larger, heavier mast and heavier antenna at 20+ feet usually requires the use of guy ropes. And those guy ropes extend the foot print and will require extra safety markers.
• Mast Supports and Tripods
  • If the station will be under a pop-up tent, then strap the mast to the tent leg with some heavy-duty Velcro for a very stable support with no tripping hazard.
  • For more portable operations, tripod speaker stands such as from On Stage Stands provide low-cost, high quality, very portable mast supports. MFJ also makes a similar tripod. When used with a lightweight mast such as the 32' telescoping mast from The Mast Company, the tripod can usually be stabilized/secured with sandbags or tent stakes. With a more substantial mast (larger surface area = higher wind load) and heavier antennas at 20+ feet, guy ropes are usually required.
  • When using tripods, and especially when using guy ropes, be sure to mark the area with safety cones and use orange safety flags/streamers on the guy ropes.