questions about trimming an antenna

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questions about trimming an antenna

Postby wmmanager » Thu Dec 06, 2007 6:18 pm

hey all, I've been using a Uniden BC92XLT handheld scanner for several months now. I started out with the rubber ducky antenna it came with. I upgraded to a rat shack telescoping model, with much better reception. My next upgrade was to a magnetic mounted 32" antenna from rat shack again. Once again, reception was greatly improved. Its mounted on the center of my vehicles roof (4 door medium sized sedan) I found a post concerning 'tuning an antenna', and chopped the 32" antenna down to 17.4", the supposed quarter wave that would boost the AAR bands. It completely sucked. my next move was to rebuild the antenna I cut and made it around 41.5" long, and reception came back to what I considered a good level for here. Since the homemade antenna looks horrible, I purchased a 64" steel whip with the intention of trimming it for the best possible results on the AAR band. My knowledge on the subject is basically trial and error so far, so I'm looking for any help that can be provided.

Just for background reference, I live in Southern WV, and I mainly listen to AAR channel 8, 14, and 20.

Thanks!

Loyd L.
Calling the CSX Alleghany Subdivision home.
Click here for my Railpictures.net photos!
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Postby clearblock » Thu Dec 06, 2007 11:35 pm

The length depends on what type of antenna you have. A simple whip antenna is best at 1/4 wavelength which is approximately 17" for the 160 MHz RR band.

You don't say what model antenna you have but if it is an all band scanner antenna it probably has some tuning components in the plastic base and it is best left as it was made. Attempting to cut it for a single band without getting rid of the tuning components in the base will only ruin the performance on all bands.

You best results are with a magnet mount base for a 17" whip or there are 5/8 wave gain antennas made specifically for the 160 MHz band that have a plastic covered base loading coil and a whip length around 48".

What model Radio Shack antenna do you have?

A good choice for a mag mount antenna is to get a "NMO" type base from Maxrad or Larsen and then you have a choice of either a 1/4 or 5/8 wave antenna that matches the NMO mount.

Railcom sells a "Railscan Gain Antenna" mag mount 5/8 wave antenna tuned to the RR band all ready to go. www.railcom.net
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Postby thor88 » Sat Dec 15, 2007 3:00 am

Unless you're transmitting, antenna length is not that important as long as you're reasonably close. Any 2m ham or VHF business band antenna will work just fine.

So figure 36" for half-wave if there's no loading coil.

If you're transmitting, that's a different issue.
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Re: questions about trimming an antenna

Postby CarterB » Fri May 08, 2009 8:17 am

Best not be trying to transmit on the RR channels. It's against the law for all but authorized users.
Full wave is best, then 1/2 wave, 1/4 wave etc. but as stated earlier, for reception, any good antenna in the 150-170MHz range should do you well.
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Re:

Postby Ken W2KB » Fri May 08, 2009 8:26 am

thor88 wrote:Unless you're transmitting, antenna length is not that important as long as you're reasonably close. Any 2m ham or VHF business band antenna will work just fine.

So figure 36" for half-wave if there's no loading coil.

If you're transmitting, that's a different issue.


If there is no matching/loading coil 1/2 wave will be a very poor impedance match resulting in considerable loss in the coax. Note that a received signal is degraded by mismatch as much as is a transmitted signal. (The only instance where a transmitted signal is degraded more is a radio with a power foldback circuit in transmit so power output decreases which compounds the losses from a mismatch)
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Re: questions about trimming an antenna

Postby Tracer » Wed May 13, 2009 8:38 pm

Very confused.... Is 17" still the ideal length to recevie 160mhz?

On longer antennas that are desighed for rr band(160mhz), is only the top 17" of the ant. recieving the signal? And is all that extra height in the antenna just for getting it higher in the air?
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Re: questions about trimming an antenna

Postby Ken W2KB » Thu May 14, 2009 3:49 pm

Triker wrote:Very confused.... Is 17" still the ideal length to recevie 160mhz?

On longer antennas that are desighed for rr band(160mhz), is only the top 17" of the ant. recieving the signal? And is all that extra height in the antenna just for getting it higher in the air?


No, half wave and 5/8 wave antennas (roughly twice to a bit more than twice the 17" in length) use the entire antenna to receive the signal. It is sometimes called a "colinear" antenna. The extra length increases the gain by about 3 decibels (twice the signal strength) and also lessens flutter in the signal resultant from the train, car or both moving. The higher in the air helps a bit also.
~Ken :: Fairmont ex-UP/MP C436 MT-14M1 :: Cessna 177B Cardinal N16019
Black River Railroad Historical Trust :: My Personal Site
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Re:

Postby Tracer » Thu May 14, 2009 4:35 pm

Is there anyway what to tell "wave" antenna i have? Its steel 17 " long steel about 1/8" thick. (its old, i dont know the brand)

clearblock wrote:
Railcom sells a "Railscan Gain Antenna" mag mount 5/8 wave antenna tuned to the RR band all ready to go. http://www.railcom.net


Would this antenna clearblock mentioned be a big improvment over the one i mentioned above?

Thanks for the help!
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Re: questions about trimming an antenna

Postby Steve F45 » Fri May 22, 2009 8:45 pm

i'de love to get that railcom antenna or the traintenna, but they are 44'' long. My truck's roof is just under 7' tall. i'de have to keep taking it down everytime i go into anykind of parking structures.
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Re: questions about trimming an antenna

Postby Tracer » Sat May 23, 2009 9:29 pm

Steve F45 wrote:i'de love to get that railcom antenna or the traintenna, but they are 44'' long. My truck's roof is just under 7' tall. i'de have to keep taking it down everytime i go into anykind of parking structures.


And no more mcdonalds drive thru. :P
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Re: questions about trimming an antenna

Postby EdM » Mon May 25, 2009 8:52 am

Trial and error cannot be bad as far as trimming an antenna. That haveing been said, any signal comparison between tries is bound to be so obliterated by vairous unknown variables as to be almost useless. 1/4,3/8, and 5/8 are the applicable measurements. 1/2 wave doesnt work without some circuitry in its base. The longer antennas (3/8 & 5/8) do do some enhancement, but at the expense of lowering the vertical angle of reception. After all the effort, a simple 1/4 wave, all considered, is the way to go. The only safe way to measure an antenna, after the ruler, is by measureing its VSWR...beyond discription here... Elsie Kay
de k2lck..spinner of antennas,tall tales and some outright lies...
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Re: questions about trimming an antenna

Postby dummy » Fri May 07, 2010 12:57 pm

its real simple and affordable. i use the radio shack magnet mount scanner antenna. it was about 40$. they give you three sections and two center loads. you assemble with an allen wrench they give you. discard everything BUT the top section. its 17 inches long. put it in the base and tighten it down. put a bead of silicone around the entry place to keep out the weather. it works real well for me. i get 15 miles in any direction. 30 or more when picking up the dispatcher off of a base transmitter. im guessing the "ham heads" will argue over better ideas but this set up works real good for me.
can usually be found along the CSX mainline between cp382 and cp393 watching,videotaping or just photograghing trains with my wife and 6 year old son.
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Re: questions about trimming an antenna

Postby Schuylkill Valley » Wed Mar 02, 2011 12:38 pm

Why spend $40 on antenna that your only going to use the 17" wire at the top and the base with coax leading out. Just take an old Car stereo antenna cut it to 17.4" and thread it on your CB mobile base and away you go. it's that easy.
I been in radio for close to 30 years.

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Re: questions about trimming an antenna

Postby EdM » Wed Mar 02, 2011 5:24 pm

Statements by most to the contrary, there is NO SILVER BULLET, or
gooder antenna. The limiting factor is noise which CANNOT be
overcome by amplification. The best antenna is a dipole or beam
which is a dipole with reflectors and directors. This topic is well beyond this
post, but is well covered for the novice by the RAdio Amateurs
Handbook, available in most librarys.. To go out and buy a
recommeded antenna by a particular manufacturer is just plain
foolish... They are all more or less the same.
That having been said:
Put a 17" (17.4) high radio shack adjustable mag mount in the center of the metal roof of your vehicle
(or on a frig or file cabinet inside), replace your scanner
(broadbanded) with a two meter amateur HT that covers RR frequencies.
Putting your HT in the center of
your vehicle roof will also enhance recption, as the "antenna" on the
HT is only 1/2 the antenna, the radio is attempting to be the other
half. (the 17.4" "antenna" is only 1/2 the antenna, if it isn't coupled [somehow}] to a "gnd", the coax will attempt to be the rest of the antenna.)
I heard this from there is so dependent upon undefined
variables that it is just ill advised..
de K2LCK, Ed... 75 grids on 144, 55 countries on 6, and over 55 years licensed,
retired RF/Antenna tech..
de k2lck..spinner of antennas,tall tales and some outright lies...
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Re: questions about trimming an antenna

Postby kitn1mcc » Thu Apr 21, 2011 6:07 pm

railcom charges outragous prices you could pick up the same thing at a hamfest or ham dealer for about 45 dollars i know from tessco the antennas (larsens) are 20 each plus a mount


hustler makes an excellent scanner antenna

http://www.pacetronics.com/group.cfm?Ca ... lected=274
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