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 Packet Radio Antennas


What kind of antenna do you use for Packet?

F

or every person that you ask, you will recieve a different answer! Hams lean toward either perfection or blue-sky dreaming, so answers will vary from the "best" one to the "ideal" one. In the end, it's whatever works for you!

Here are some general guidelines that I have found helpful over the years. If you are new to Ham radio or new to Packet radio, it is probably better to buy your first antenna as opposed to building it yourself. I know this contradicts the spirit of this site, but to get your station up and running in a reliable fashion, it is probably safer to buy. This approach also gives you a base line from which to compare your homebrew antennas which you will build in the future!

Vertical or beam? Again, it all depends. If you are in a remote area and have no nearby radio neighbors, then it has to be a beam aimed at your only neighbor. This might be your main node or switch though which you relay traffic to all your contacts. The beam gives you the best chance by concentrating all its power in one direction. I use a beam to test connections to new or distant stations, or to stations that I don't routinely contact.


A vertical antenna works well when you have lots of neighbors who are fairly nearby, thus forming a kind of wireless hub-and-spoke network. (Note too, a vertical transmits equally in all directions using vertical polarization, an obvious but important point.) It is also usually easier to place a vertical at a much greater height than a beam from a purely logistical standpoint. (Verticals have far less wind loading than beams.) And, as with all antennas, height is everything! So, the odds tend to favor verticals...

I use a vertical (ground plane) for my 145.090 frequency since I can reach other stations in almost all directions within a 15 mile radius. So by hopping from these radio nodes, I can effectively extend my range to up to 50 miles or more. (Also, my monitoring of the frequency with the vertical can show a very large number of stations on the air whereas my beam will rarely show more than 3 or 4.)

Here is one place where verticals can give you misleading information. Verticals can "hear" very well, but that doesn't mean that they have enough power to reach all the stations listed in the "heard" list. One reason for this is the omni-directional transmission. It tends to dilute the power by spreading it out. A beam, on the other hand, can almost always reach the stations it hears... Ah, decisions, decisions...

Generally vertical antennas come in three "flavors," although there are many varieties of this class of antenna: the center-fed vertical, the ground plane vertical, and the stub-tuned, or J-Pole, vertical. The center-fed and the J-Pole offer the lowest angle of radiation. This means that they will transmit, on average, over a greater distance. However, both types are free space radiators, making then easily detuned by nearby objects, particularly metal objects, even their own feed lines! The ground-plane is less sensitive to this type of coupling, but its angle of radiation is not as low as the other two mentioned above.

The J-Pole, while a great choice for any radio system, is tricky to build, probably much more difficult than the other types of verticals because the adjustments on the tuning stub must replicate as much as possible the real conditions under which this antenna will be transmitting... (But tricky as they are to build, I really like J-Poles .)

Another choice for an antenna is the loop. There are single loops and multiple loops, otherwise known as a beam. The single loop is effective for connections along an axis, for example, North and South of your location. Make sure that you feed the loop on its side to obtain the correct vertical polarization since all VHF packet transmissions are vertically polarized. If you would like to see if you have room for a loop and what type of design might be best for you, link to the Loop Calculator Page... Figuring Practical Loop Designs

Beams made of loops are quite popular for voice work, but have application for packet as well. I have a 4-element loop beam (a quad) in my attic and it works quite well. It is a homebrew project and was easy to build and quite inexpensive. There is a larger picture of this antenna on the home page of this site with a link to a general description of how to make one... My Quad in the Attic

A very popular antenna is the Yagi, usually of 3 or 4 elements, although they can be up to 11 or more. ( Again, make sure that the elements are positioned in a vertical alignment to achieve the correct vertical polarization for VHF/UHF packet.) Gain for this antenna ranges from 4-5 db to 10 db or more, depending on the number of elements. It can be used as either a fixed antenna or a rotatable one. Most hams purchase this type of antenna since its popularity has brought the price well within reach of the average ham. (But, it is not difficult to build if you want to try your hand at the classical "boom and element" construction techniques.)

Another very interesting and somewhat unusual antenna is the Quagi. This hybrid combines elements from both the quad and Yagi antennas. It seems to find more application at the UHF frequencies, though, probably because it is easier to feed the driver as a loop as opposed to a vertical element. The back two elements, driver and reflector, are quad loops while the directors are verticals like the Yagi. Its use is very similiar to the Yagi, a high gain beam which can be fixed or rotatable.

An antenna that is less frequently used, but still offers interesting possibilities, is the Discone. This amazing antenna has a bandwidth of up to 8 octaves! So you really never need to use a tuner with it. However, on the down side, it has no gain. Many people use this antenna for scanners or FM Commercial reception, but it can be used quite effectively for packet or voice VHF as well.

And all of these VHF antennas should be fed with the lowest loss coax cable that you can afford. Sometimes, you can feed them with 300 ohm ribbon line, but these types of antennas are usually of the homebrew type and frequently require matching devices at both ends of the line... 50 Ohm coax is the standard for today's use!

What about HF packet antennas? Although there is not a great deal of activity on HF packet, there is more than you might think. And again in general, there is no particular antenna that will present any special advantages over the normal ones used for voice transmissions, such as: the dipole, the loop, the J-pole, and the beam. Any of the above that can be used for voice, will work equally well for packet. In fact, many HF packet operators share their entire radio station modes of operation, including the antennas, switching between voice and packet as occaision demands. (A very versatile and inexpensive antenna is the multi-band horizontal loop which works quite well on 14.105Mhz LSB, the Network 105 frequency. Here's a calculation sample in real time...)

As with all outdoor antennas, make sure that you have a ground system that will carry static build-up and lighting strikes to earth, especially if you have a high vertical mounted near or on your house!

So, when all is said and done, you have to decide what antenna is going to do what job; what is the capability of each antenna, and with whom each antenna is going to communicate. Whether you have 5 or 6 running, or are just putting up your first one, the antenna is the front-end of your communications system, and perhaps its most important single component. When you can answer all of the above questions, you will know the best antenna(s) for you and your station.



(Courtesy KBNorton Computer Systems)