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Railfan Technology > Build an antenna for railfanning


Date: 06/03/22 19:31
Build an antenna for railfanning
Author: wa4umr

Occasionally there are questions about antennas for monitoring the railroads.  In the past, I have recommended a few commercial models, but some can be built if you have a little skill.  The "J-pole" antenna is loved by some and hated by others.  It's easy to build because it uses 1/2" copper pipes and fittings and a little solder.  I haven't tried using any of the solderless connectors nor have I seen anyone else try them.  They may work, but I just don't know.  You also don't need any insulators to be fabricated.  The antenna electrically is one piece.  It can be grounded for lightning protection.  

I've done the arithmetic for you and put them in easy-to-measure units.  Since you are not transmitting, the measurements are not extremely critical.  

A = 62-1/4"
B = 17-3/8"
C = 1-3/4"
D = 1-5/8"
The piece below the T is not critical, but 12" to 24" would be typical to provide a secure mounting.  Use 2 or 3 hose clamps to secure it to a mast.

Make sure you use some steel wool or Scotch-Brite to clean the connections and some flux to prepare the pipe for soldering.  You might want to use some copper pipe clamps to configure a connection for the coax.  Make sure you weatherproof the end of the coax to keep the rain out of the end.  

I won't go into detailed construction ideas.   I figure that if you are interested, you can come up with some creative techniques.  

John





Date: 06/04/22 08:38
Re: Build an antenna for railfanning
Author: WW

One of the biggest mistakes that I've seen made in permanent antenna installation is improper grounding--and it's regulary made even by professional radio people.  It is this:  Driving a ground rod and grounding the antenna to it is NOT sufficient.  Why?  Because, if the radio is grounded to a different ground source--say, the grounding of the building's electrical ground (usually found near the electrical service connection and disconnect) and that grounding source is not tied to ground rod of the antenna, an electric potential is created.  If lightning strikes the antenna, it can fry the radio and other electrical equipment because of the two grounds not being bonded together.  I remember one commercial installation where the base radio in an lightning-prone area was getting fried at least once a year.  Finally, a electrician discovered that, after some foundation work, the copper  bonding wire between the antenna grounding rod and the building electrical grounding rod had been severed, not repaired, and covered up.  Once the two grounds were bonded back together, the site went for years without lightning-caused radio damage.



Date: 06/04/22 11:16
Re: Build an antenna for railfanning
Author: TCnR

Agree with the love / hate description. As mentioned by the OP, considering it as a receive antenna makes it easier to understand.

Somebody was good enough to write a wikipedia with some history and technical description:

https://en.wikipedia.org/wiki/J-pole_antenna



Date: 06/04/22 19:04
Re: Build an antenna for railfanning
Author: wa4umr

WW, you are absolutely correct.  I worked at the phone company and in our buildings, EVERYTHING went to a common ground.  If there were multiple switches in a building, they all were tied to the same ground.  The safety ground on the distribution frame, the transmission equipment, the power plants, the equipment racks, and EVERYTHING went to a common ground.  Even a small lightning bolt of 10,000 Amps with 0.1 Ohms between two points, could create 1000 Volts potential between those two points.  A little more resistance and you are truly talking about some pretty high voltages. 

BTW, I've never seen the ground for one of our buildings but I understand that it is a pretty gigantic hunk of copper.  A copper thief could make a real haul if he could haul it off.

Thanks for your input.

John



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