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Scanner / Receiver Antennas For discussion of any type of receiving antenna used by a scanner or receiver base, mobile or handheld.

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Old 08-06-2005, 11:29 PM
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Default Antenna lengths per band

Anyone that can help me out with this information will be greatly appreciated.

I understand that no one antenna can cover all the bands clearly so I was thinking about using multiple antenna tuned to specific ranges.

I need to know how long a low band antenna covering the 46.000 range needs to be.

I need to know how long a TRUE VHF antenna needs to be, covering the 150.000 ranges.

I need to know how long a TRUE UHF antenna needs to be, covering MED ranges (462.950 - 467.950)

I also need to know what kind of wiring I need to use. I know there low-loss wiring out there and I'd like to use it because I'm very into technology and the room where my scanner is in has multiple computers and I need as little interference as possible. Something about LMR-400???

Does anyone suggest building or buying a single-shaft antenna (like that is on a fire truck or ambulance) instead of one that has multiple "legs"? I would like to make or buy one for each band.

Also, what type of connector interface box do I need to get to have them all connect to one box and then have one main wire run to the scanner for the "antenna in"? I'm not sure what to call it but I'm guessing a junction box or something of the sort. Should it be powered or have a signal enhancer in it?

I would like to put these antennas on the roof of my house but if I can't do that, they will be in the crawl-space of my house. Any ideas and/or suggestions, please either reply, PM me or email me direct: thegrimreaper@nmax.net .

Thank you for your time.
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Old 08-06-2005, 11:47 PM
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Ok, well I found a page that told me how long each antenna needs to be at full length, 1/2 and 1/4 length. Since I can't really have them at full, I'd prolly use 1/4 legth which is what most fire and ambulance rigs use.

My results:
Low band - 20 feet (full length) - 10 feet (1/2 wave) - 5.0 feet (1/4 wave)
VHF band - 6.0 feet (full length) - 3.0 feet (1/2 wave) - 1.5 feet (1/4 wave)
UHF band - 2.0 feet (full length) - 1.5 feet (1/2 wave) - 0.5 feet (1/4 wave)

If any of this information is wrong, please let me know so I may adjust lengths accordingly to band.

As far as the wiring and connectors and whatnot, I still need info about that.
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Old 08-07-2005, 12:14 AM
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Quick and easy calculation to use to find that information:

984/freq in mhz = full wavelength
492/freq in MHz = half wavelength
234/freq in MHz = quarter wavelength

You will most likeley be intrested in the quarter wavelengths. The best way to do this is to find the middle of each of the frequency ranges you want to hear and use that for your calculation. For example, if all of the VHF-HI comms in your area are between 152.0-165.0 you would want to use 158.5 for your center frequency and cut the antenna for that length (234/158.5=1.47ft or about 17 inches) and so on. I posted a generic list of measurements in another thread about a wire antenna, the guy who made the antenna said it works great. Another thing about antennas is that not only they receive the ranges they are "cut" for but also receive multiples of that range, for example, a quarter wavelength antenna cut for 450MHz can also be used as a 1/2 wave antenna for 800/900Mz and a quarter wavelength antenna for 50MHz can be used as a 3/4 wavelength antenna for 150 MHz. When you get to UHF and 800MHz frequencies you can use a single antenna for both ranges with great results. In general, 1/4 wavelength antennas are well suited for scanning and they will have a similar receive ability as a 1/2 wave dipole, and a 1/2 wave dipole is just a 1/4 wave leg attached to the center conductor and another 1/4 wave leg attached to the shield. As for connectors, you can build a multi band scanner antenna for 150, 450 and 800 MHz using a simple panel mount SO239 or other similar RF connector and some stiff copper wire. here is a good reference for building your own scanner antenna

http://www.scancat.com/vhf-ant101.html

below is a pic of a 450/800MHz antenna I made yesterday from parts I had here in the shack. Also, you can use LOW LOSS CATV type splitter/combiners like the one the cable company put on the side of your house to split the cable TV signals. I have one mounted on my mast and have my scanner antennas connected to it and just one feed line coming into the scanner. Make sure you use a combiner that is rated for 1-1000MHz or more so you get the best signal at the higher frequencies.

http://www.azuma.ath.cx/kf4lne/stuff/450-800.jpg
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Old 08-07-2005, 12:02 PM
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Post Scanners and computers = poor RF mix

Quote:
Originally Posted by SlipNutz15
I also need to know what kind of wiring I need to use. I know there low-loss wiring out there and I'd like to use it because I'm very into technology and the room where my scanner is in has multiple computers and I need as little interference as possible. Something about LMR-400???
You're right to be worried about the computer/scanner mix. Keep the antennas and scanners as far away from the computers as possible.

The computers put out a fair amount of noise. This causes two problems... 1) noise on the frequencies you care about and 2) strong signals on other frequencies getting into the scanner and "desensitizing" it for the signals you care about...

Distance is the best preventative measure. Shielding / ferrite beads / etc is a poor second.

Plec
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Old 08-07-2005, 12:40 PM
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Quote:
Originally Posted by PluckyPleco
You're right to be worried about the computer/scanner mix. Keep the antennas and scanners as far away from the computers as possible.

The computers put out a fair amount of noise. This causes two problems... 1) noise on the frequencies you care about and 2) strong signals on other frequencies getting into the scanner and "desensitizing" it for the signals you care about...

Distance is the best preventative measure. Shielding / ferrite beads / etc is a poor second.

Plec

Very mucg agreed. Also, to keep noise out from the scanners power cord you may want to put a ferrite bead on the power cord at the back of the scanner. I once worked in a radio station and I had to eliminate some noise from the computers that was leaking out of the computer into the building electrical system. Not a pretty sound
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Old 08-07-2005, 1:39 PM
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Where can I get that farite bead that you speak of? and price?

The frequencies I'm scanning are:
Low band - 46.060, 46.100, 46.160
UHF band - 460.600, 460.625, 462.950, 467.950

Thats why I want to use two antennas for each of the bands desired.

As far as the connector box, I'm assuming I can get one at radioshack for pretty cheap? One that makes multiple connections into one

Also, on that site that says how to build an antenna, it says the thinner the guage the better for reception? Is this correct? Should I use a 1/16 wire or 1/8?

Did some searching. Is this that ferrite thing?
http://www.radioshack.com/product.as...t%5Fid=273-105
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Old 08-07-2005, 2:38 PM
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The thinner the guage the better the RX is questionable at best. The wire you use will be energized best as long as it is "tuned" to the frequency range you want, meaning it is resonant. The combiner can be picked up at radio shack for a few dollars depending what you get, do NOT get the one for 18 inch dish, they do not work well below 900MHz. For best results you may have to comprimise with the materials between electrical effeciency and structural stability. I would suggest using brass tubing from a local hobby and craft shop for the antenna elements on the low band antenna and good, stiff 10 or 12 guage copper wire for the high band and uhf antennas. Now if you want a REALLY good antenna system for the scanner I can point you to a website that tells how to build them, but those antennas are for the band they are built for and barely work at all for other bands so you have to build an antenna for each band you want to hear and they require a bit of antenna theory knowledge to go along with them. As for teh ferrite bead, go to radio shack and tell them you want a snap on ferrite choke, it is usually in the "parts" section


And yes, that is the ferrite choke
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Old 08-07-2005, 2:53 PM
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Talking

You have been given some good information in these posts. If you want to understand the "why" of all this I will explain. Radio waves travel at the speed of 300 million meters a second (same as light in a vacuum) or 984 million feet per second. Frequency is the measure of how many wavelengths per second. So, if we divide the speed of travel in seconds by the number of waves per second we get the lenght of one full wave (or cycle). Thus 984 (million) divided by the frequency in megahertz (million) gives us the length of one full wave. But antennas are usually a 1/4 wavelength so taking 1/4 of 984 equals 246. Thus 246 divided by the frequency in megahertz gives us the distance for a quarterwave antenna in feet. Simple

Last edited by northzone; 08-07-2005 at 3:00 PM..
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Old 08-07-2005, 2:54 PM
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Well I am planning on building multiple antennas for the separate bands.

As for the materials for the individual antennas, either reply or PM me the materials that I should use. Is it ok to use the SO-239 for the connector for both low and UHF antennas or is there separate connectors for each band?

I'm a little confused what you meant by "I would suggest using brass tubing from a local hobby and craft shop for the antenna elements on the low band antenna"

Brass tubing? Explain that one please?
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Old 08-07-2005, 3:20 PM
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Hi Slip and readers,

All bases except the most important one were covered, now to attack that one. Before you even attempt to build an antenna you must learn about design. Just knowing how long to cut the elements by fractional wavelength is the tip of the iceberg, it varies with the type of antenna you want to build. Groundplane, dipole, discone, beam, what? Then there are the exotics like pylons, slots, loop Yagis, Sterba curtains, helicals, fractal panels, etc., it'll make your head spin at least until you read up on the subject.

There are all sorts of technical publications out there, far too many to mention but checking out www.arrl.org is a great place to start. Meanwhile, if you have a specific design in mind we can help you.
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Old 08-07-2005, 3:25 PM
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I'm planning on just doing the SO-239 design with the main receiving shaft out of the center vertically and the ground receivers coming out of the corners at 45*. If you would like to talk to me, email me or AOL IM me or just reply. I'm very interested in this and if I could get into some detailed discussions, I'd appreciate it
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Old 08-07-2005, 4:21 PM
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the reason for using brass tubing for teh vertical element is because brass tubing isnt as flexable as coper wire is and it will hold up better in the wind and because it is brass you can still colder to it. DO NOT use aluminum or steel, you cant solder to those metals. use a small brass tube if you go that way and you can use a brass brazing rod to extend the tube for the low bands. Once you have teh antenna built and you are using it you can feel good that you manufactured your own antenna that is comparable to commercially produced antennas I have only purchased 3 fixed station antennas in my life, the rest were built from scrap TV antennas or various metal tubes and such. I only buy mobile antennas because I don't have a machine shop to make a mobile antenna that will hold up to daily driving
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Old 08-07-2005, 4:38 PM
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So I should use brass tubing to go how far vertically with the low-band?

Also, how do I solder it to the SO-239? Since I can't actually have anything that goes into the center hole? Any ideas/tips/tricks?
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Old 08-07-2005, 5:08 PM
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On one side of teh SO239 will be the threaded part where teh coax connects and on teh other side will be the solder tabs. Just put the vertical element in that center pin on teh solder side of the SO239 and solder it. As far as using the brass tube for the low band antenna, use at least 3 or 4 feet of it, that way you have a good, sturdy antenna that will be more resistant to bending in the wind. Just to give you an idea of how far you can go with solid copper wire before it is too long to use for an antenna, take a length of wire, hold out horizontally. If it starts to bend then its too long to use by itself and you will have to find something that is made of a more rigid material, like brass tubing. You may also have to use brass tubing or something moe rigid than wire for the low band radials too since they will bend when they get that long, I suggest using brazing rod and brass tubing, you can use teh brass tubing to extend the length of the brazing rod by cutting the brass tubing in lengths of about 2 inches and inserting the brazing rod into the tubing and soldering that with a propane torch or a really hot soldering iron. Brazing rod can be picked up at welding supply shops and is typically made from a copper or brass alloy that can be easily soldered to, basically brazing rod is a welding rod used for copper and br**** commonly used to "braze" oxegen lines and other such types of plumbing as well as used in copper art work. Also commonly used during the installation of ground planes at AM radio towers, not a fun task in the middle of July since AM radio tower bases do NOT have any shade beyond the support for the base insulator. I know this to be true because a few years ago before WTZK became WWRN and moved TX locations I was involved in the TX site preeparations and transmitter installation. That was one hell of a feat, the antenna was a 50.13 ohm load at 1.35MHz We were happy because that meant we didnt need a very exspensive coupling box (the dog house at the base of the tower)

EDIT: had to show the pic of the monster. i took this pic while taking a break in the shade at the bottom of the tower. That antenna is cut for 1.35MHz and is around 180 ft tall

http://www.azuma.ath.cx/kf4lne/stuff/DCP_0048.JPG
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Old 08-07-2005, 5:49 PM
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Do I have to use radials for the low-band antenna or can I just have one main shaft?

Let me see if I understand this. I need to get a piece of brazing rod that will fit into the center solder point and the brass tubing just keeps everything in place? Does the tubing top and the brazing rod have to be soldered at the top to make one big antenna or what?
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Old 08-07-2005, 7:15 PM
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Put the brazing rod sections into the brass tube sections and solder together to make one long shaft, i think brazing rods come in 24 inch lengths and brass tubing in 36 and 48 in lengths. You cut the brass tube about 2 inches long and use it like a butt connector to extend the length of the antenna, do teh same with teh ground radials. Ground radials are not required, but they do offer feedline decoupling and help keep the coax from being part of the antenna, basically teh ground plane radials keep the coax and antenna from seeing each other
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Old 08-07-2005, 7:30 PM
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Well I was planning on using LMR-400 for everything so there wouldn't be an interference. Also, like on my local fire truck, we use lowband and they have antennas that are like 3.5 feet long? They pick up very well. Is that a possibility?
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Old 08-07-2005, 8:04 PM
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Using LMR 400 still exibits the same need for feedline decoupling as does any other coax. The ground radials will cut down on interference picked up because of the coax becoming part of the antenna, the radials are basically a wall that keeps the vertical element from benig able to induce stray RF on teh shield of the coax. As for the 3.5 foot antennas for low band, I am familiar with those antennas, I have one for my 6 meter mobile install. In the base of the antenna there is a coil of wire that makes up for the difference, this coil of wire has to be made a certain way or it wont work right and the antenna will not function. Same way CB antennas work, they have a coil of wire to make them electrically 1/4 wave length long when they are actually much shorter.
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Old 08-07-2005, 8:09 PM
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Ok... is it advisable to try to make one of those shorter 3.5 foot antennas?
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Old 08-07-2005, 8:29 PM
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Don't use aluminum? It's my main course!

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