Railroad band pre-amplifier...

[jjmdimunno]

Hey guys,
I run a railroad radio stream for Southern New Jersey, and am looking to boost some of my weaker signals a little bit...can anyone here recommend a nice preamp to use for the railroad band? I don't want to use a wide-band amp, since that'll amplify everything, including the junk I want to get rid of.

ANy recommendations would be welcome!

Mike DiMunno

 

[gcgrotz]

You will probably need a bandpass filter in front of the preamp, or use a yagi if you are not already. Advanced Receiver Research makes some great preamps. I assume (hope) you are not near 158 MHz paging or NOAA transmitters.

 

[k9rzz]

I don't think you're going to find a box labled "VHF RAILROAD PRE-AMP", so I guess there's a number of ways to do this.

- Build one. There's a number of websites describing how to do this. It wouldn't be too hard, but I suspect this isn't what you had in mind.

- Buy a commercial VHF pre-amp cut for the frequency you specify. It will probably work great over the range you need. I doubt it will be cheap though. What's your budget?

- Use a broadband pre-amp and if you get some intermod from a loud transmitter, add a coax stub to your feedline via a T connector to notch them out. It might take a little trimming, but could do the trick.

- Use a combo of the above: broad band pre-amp plus home brew band pass filter. Again, not that hard to do, but will take a few hours of tinkering to dial it in.

John K9RZZ

 

[OpSec]

- Use a broadband pre-amp and if you get some intermod from a loud transmitter, add a coax stub to your feedline via a T connector to notch them out. It might take a little trimming, but could do the trick.

I have a question...I need to notch out a NOAA transmitter,various 152/153 MHz data freqs and a 154 MHz paging channel (local FD) that are killing most of my VHF channels from 144-161 MHz. I do not need to use a preamp, as the desired signals are strong enough, and I need to listen to 150-159 MHz for work purposes. Can I build a tuning stub or stubs for this?

Any links to the theory behind this so I can read up on it?

*edit*

Did some research and came up with this 1/4w stub calculator

"To calculate the length of the stub in inches use 5904/f, devide by two and then multiply the length by the velocity factor (VF) of the coaxial cable. To find the VF of a cable you will need to look it up. The ARRL handbook is a terrific reference for this. Most RG-8 and RG-58 type cables have a VF of .69 while Teflon cables tend to be higher. The air dielectric cables are higher yet. The insulation around the center conductors actually slows the radio wave down and that is known as velocity factor. You can also go HERE (link on his page) to see a VF listing of the more common cables."

I made one to specs for the pager transmitter, and it cleaned up my NOAA problem as well. I still have a little problem with the data, but it's from our system on the same tower as the scanner antenna. I don't think the stub alone is notching the data freqs deep enough since the stub is only -25 to -30 dB.

 

[k9rzz]

Perhaps a stripline band pass filter (build your own) would be narrow enough for your application. I also noted that there are band pass filters advertised right on the home page of Radioreference.com !

John K9RZZ

 

[k9rzz]

Here's some links I found while surfing and eating lunch:

http://cgi.ebay.com/Pico-Macom-BPF-...Filter-5h_W0QQitemZ170086953191QQcmdZViewItem

http://www.dci.ca/?Section=Products&SubSection=Bandpass

http://www.ifwtech.co.uk/g3sek/swxfiltr/swxfiltr.htm

http://www.starkelectronic.com/btp52.htm

That's all I can offer at the moment. My current problem is changing out the brakes and struts on my Subaru Outback !

John K9RZZ

 

[gcgrotz]

With the death of 2way radio systems, perhaps a local shop would have some cavity filter they would re-tune and sell cheap. I have a big problem with nearby paging from 158.100 and I bought a PAR notch filter. It works great but the signal from 1/2 mile away still gets through some at 160 Mhz.

 

[kb2vxa]

Hi Mike and readers,

"With the death of 2way radio systems, perhaps a local shop would have some cavity filter they would re-tune and sell cheap."

Even a used cavity resonator is ANYTHING BUT cheap and FAR too narrow for your purpose. Oh, I almost forgot huge like a tall beer keg and a bandpass filter requires several interconnected cavity resonators. Perhaps Mr. Grotz meant a helical preselector scrounged from an old transceiver. I ripped a 5 section unit out of a Motorola President, a real POS radio but the 10dB gain and 1MHz wide (500KHz each side of center frequency) selectivity skirt was ideal. Why Frankenstein it when a junk rig has just what you need sitting inside?

On the other hand why not do it right like the big boys on the streams do? With a bit of luck someone just might drop an old Syntor or Maxtor in your hot little hands that'll pee all over any scanner ever made.

 

[N_Jay]

. . .. a Motorola President, . . . .

A what?

 

[mancow]

Also, what is a Maxtor? I thought that was a hard drive.

A Maxtrac maybe?

:wink:

 

[k9rzz]

Here's some prices (new) for kicks:

http://shop.electro-comm.com/ac/Home.aspx?view=78

John K9RZZ

 

[gcgrotz]

Hi Mike and readers,

"With the death of 2way radio systems, perhaps a local shop would have some cavity filter they would re-tune and sell cheap."

Even a used cavity resonator is ANYTHING BUT cheap and FAR too narrow for your purpose. Oh, I almost forgot huge like a tall beer keg and a bandpass filter requires several interconnected cavity resonators. Perhaps Mr. Grotz meant a helical preselector scrounged from an old transceiver. I ripped a 5 section unit out of a Motorola President, a real POS radio but the 10dB gain and 1MHz wide (500KHz each side of center frequency) selectivity skirt was ideal. Why Frankenstein it when a junk rig has just what you need sitting inside?

On the other hand why not do it right like the big boys on the streams do? With a bit of luck someone just might drop an old Syntor or Maxtor in your hot little hands that'll pee all over any scanner ever made.

Hi Warren:

Yeah that too, I did mean cavity filters though. I know they are big and I personally don't have room for them but someone might. I used to work on GE equipment and the helical resonators on the MastrII were pretty tight. They would make a nice filter.

I have used 1/4 wave stubs too. You need a spectrum analyzer or something to get them tuned really well, and they are fairly broad but they will give you 20db or more of notch. Just watch out for the odd multiples, they will get notched out too.

I mentioned PAR already, his website is kind of low tech but his filters are great. The 158 notch I bought knocks out close to 60db at 158.100 and I can still hear railroad stuff at 160 well enough to keep up with things.