Antenna

There has been a lot written and discussed about antennas for scanning. The one thing you first have to understand is that there is not the 'perfect all around antenna' because it depends on what you are trying to listen to, what type of equipment you have, is there strong RF in your area and a number of other factors. No one single omni directional antenna could do real justice to the wide area of spectrum that today's scanners can receive. You can wish all you want but the laws physics just say it won't happen.

When you consider or use a 'wide band' antenna you are doing a trade off of gain and rejection for a wider range of received frequencies. That is not always bad but you need to be aware of it and understand it to maximize your scanning performance. There may be times that a 50 – 2000 MHz omni directional antenna may work great if you are in a large metropolitan area for example and need to do general scanning of everything around. There are other situations where you need or want something different; something that will give you improved reception, and rejection of unwanted signals.

Lets talk about the most common types of antennas, and here I am primarily looking at outdoor antennas. The one a lot of people like to use is the discone. Understand one thing about this antenna, it is wide band, it will receive at about unity (no) gain across a wide range of frequencies. That is good and bad, while it receives the frequency ranges you want it provides no or very little gain and it also receives those frequencies you don't want and that can overload your scanners front end like broadcast FM and pager transmitters (or other high power transmitters that are close to you). The key thing you have to remember with a discone is that there is no gain but not a lot of loss (or rejection) across the whole band that it is designed for. Some discones have a loaded whip that will extend its range down in the low VHF area as well. When you start with no gain and add the loss of your coax (see more about coax and loss later in this page) you end up with a signal to your scanner that is quite weak compared to what it could or should be. Given, it is consistent over the frequency range the antenna is designed for but weak. This is not to say that a discone will not work for you depending on your location. Many use these with good results.

I use an Icom discone with the loaded whip as a good, general antenna. I have a preamp in line to help overcome some of the loss and a broadcast band FM trap in line as well and it does a fair job. It does bring in some noise from pager transmitters close to me and without the broadcast FM filter the VHF air band is descended almost to the point it is not usable. They can work but don't expect too much form them. Single band antennas, with some gain provide a better answer to your scanning needs. You do have to switch antennas in and out of your system and that is a big drawback. Some have found that using a wide band antenna like a discone for general scanning and then switching the single band, gain type, antenna in when listening to something specific works the best. Others have dedicated separate scanners to a single band and run multiple scanners at the same time. Still others use the antenna cut for the specific band they are most interested in or have a problem receiving and let the other local signals (in the other bands) suffer but they can still hear the local stuff.

You can build or buy a number of good gain type antennas for any of the bands that your scanner will cover. Some find ham band antennas work reasonably well for example they will buy an antenna for the 430-450 ham bands and cut it down for the 460 MHz band. if you want to try a good omni single band antenna that is easy to build and will not cost you much you will want to consider the J-Pole. When you use a single band antenna you get gain on the band you want and get rejection (not total lack of ability to receive) in the other bands. This helps by bringing stronger signals from in the band you have the antenna for and reducing the overload and noise from out of the band. It gives you a double benefit.

Another type of single band antenna is the beam. In addition to bringing stronger signals from in the band you have the antenna for and reducing the overload and noise from out of the band it also gives you rejection of all signals in the direction the beam is not pointing. That means less noise, less interference and stronger signals of the transmissions in the band the antenna is cut for and in the direction it is pointing. A beam offers more gain normally than the omni antenna and this is another plus to you for a better signal. These work the best if you are outside town or have a specific direction you want to hear more from.

A good multi band beam type antenna I have found is the Log Periodic and you can find them from about $57.00. They are 'TV antenna' type construction so not the strongest in the world but they do work well, especially in the 400 - 500 MHz band. This might be a good compromise if you want a wide band antenna with performance. Unless you are outside town and are interested in signals from only one direction you will need a small rotor to turn the beam. It will pick up local stuff off the side or back of the antenna but maximum efficiency is off the front where the antenna is pointed. This antenna will also have some tendency to cut down stronger signals (other than the way it is pointed) from strong FM broadcast and pager systems to help overload.

Some General Tips

No matter what antennas you use, here are some general guidelines I have found to work the best. Get this antenna on your roof or tower, about 20 feed up to clear near by trees and houses it will work fine, much higher will only help a little and coax loss can be a issue so I suggest about 20 feet above your house as the best location. Keep the antenna away from trees, metal and wires; mount it in the clear as much as possible. Some (and yes I do have some as well) people mount their antenna in the attic of their house. There are a couple of points that I need to make about this. First, you are not achieving that clearance of near by trees and houses so the trees and other objects will affect the performance. Second, you have to watch the type of roofing material used, if it is metal the antenna will not work well period but if it is conventional shingles you will notice a significant attenuation of signals during rain (or snow) coverage of the roof. This is especially true in the UHF and above bands. The third issue is that you can pick up a lot of interference from the electrical wiring in your attic (as well as houses close to you). The bottom line is many do use this kind of setup but there are problems with it, nothing will beat being 20 feet above your roof (or clearing the near by trees and houses) with a good ground system.

Use Belden 9913 or Time Microwave LMR-400 Coax on all of the frequencies, the line loss on the lower frequencies is not bad but it is a real issue at higher frequencies this is not the place to try to save, use 9913, LMR-400 or hard line if you can. Avoid using adapters they cause a great deal of loss and are the cause of a lot of system failure. Plan and put the proper connectors on your coax. If you must use adapters buy good ones you will be much better off in the long run. I do not suggest using splitter or combiner. The losses on a splitter are too high and they take back all the gain you make with these antennas

Make sure you ground your antenna for lighting protection and to help reduce noise, and cut down on over load. and you should consider good grounding. It is an important part of any outdoor antenna system and will help give you better results as well as protect your system. Remember, no grounding will save you from a direct or near lighting hit, disconnect your antennas when not in use.

Coax and loss

First what do all the numbers mean. Loss is given in db and as a simple guide you can consider 3 dB is twice or 50%, 10 dB is about 90%. So if you have a 3db loss you will only receive ½ of the original signal power (received at your antenna) into your scanner. Here are some quick conversions from db to percentage of loss: 1db =20% 2db = 36% 3db = 50% 4db = 60% 5db = 68% 6db = 75% 7db = 80% 8db = 84% 9db = 87% 10db = 90% 20db = 99% and 25 db = 99.7% loss.

In general the longer your coaxial cable length is from the antenna to your radio, the more attention you must give to the type of coax your using. Even the best antenna in the world will never reach its full potential if the signal doesn't make it to the scanner. Use 9913 or LMR-400 coax on all of the frequencies, the line loss on the lower frequencies is not bad but it is a real issue at higher frequencies this is not the place to try to save, use 9913, LMR-400 or hard line if you can. Avoid using adapters they cause a great deal of loss and are the cause of a lot of system failure if water gets into them or they go bad. Adding connectors can cause 1 to 2.5 db loss per connector at higher frequencies.
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You can see a chart of loss with many type of coax at: Link to loss chart remember a loss of 3db is about ½ of the signal lost in the coax. Greater than 3db loss is more than ½ of the signal your antenna receives in your coax before it gets to your scanner.

What else can I say? At 900 MHz the loss factor in RG58 is 27.9 db per 100 feet (and you wonder why you can't hear that 860 MHz trunking system very well) while the loss with 9913 coax is only 4.2 db at 900 MHz (a little over 60% loss per 100 feet). These are huge losses and no mater what you do with antennas you cannot over come poor coax. Use the best coax you can find and keep your run as short as possible. Do not use runs of shorter coax with connectors and adapters, adding connectors can cause up to 1 to 2.5 db loss per connector at higher frequencies.

Consider if you are using a discone with 0 db gain and then add 27.9 db loss to it from 100 feet if RG58 coax (at 900 MHz) and you will be lucky to hear much at all. Use the best coax you can find, 9913 or LMR-400 minimal and hard line if you can.

I have had some tell me that their scanner-mounted antenna does a better job than the outside antenna they just paid good money for. Well that could be true given the factor of utility gain antennas and great loss in the coax (or could be due to overload of the scanner front end by strong transmitters) but if you follow the information above you will have much better luck with your antenna installation and performance.

Over Load

The wide band front ends of a sensitive scanner can be overloaded by strong signals. If you put up an outside antenna and have good coax but your signals are not as good as they were before try to turn the attenuator on (if you have one) and see if that helps. If it does then there is also a good possibility that your scanner is being 'overloaded' with too much RF from other nearby transmitters.

Over load happens to a receiver that is receiving so much RF that it begins to seem like it has lost any sensitivity is being 'desensitized'. You have to try to find out what is causing the problem. If you have a local broadcast station it could be causing vhf high band desense problems in the air or weather radio band. Try the $4.99 FM trap from Radio Shack (15-577). I was amazed with the difference on a PRO-2053 connected to a Scantenna.

There are filters to help reduce the problem of pager or high power transmitters in a specific band, do a search on the internet and you will find a number of them from people like Par Electronics . You can also build a simple filter to help reduce the problem. The one that comes to mind is a ¼ wave stub notch filter. A simple stub-filter placed in the coaxial cable may help. This comprises of a `T' connector with an open circuit 50-OHM cable length (the stub) attached to the `T' piece. A rough calculation for the stub length is: (75 / Freq in MHz) x 0.67 = Stub length in meters. )

I like to run a ground wire to each of the scanners and ground the case (or the metal that is grounded) on the scanner. I think this helps reduce direct overload to the scanner and may help reduce problems if you transmit near the scanner with another radio. I think it is good electrical safety to have each receiver / radio earth grounded. This also helps reduce interference and noise from computers and other electrical items in your house.

Remember that this is NOT a defect of the scanner; it is a sensitive, wide band receiver that you are feeding too much signal from an unwanted source to. Read the information above about a single band antenna and how that could help with over load as well.