building a 5/8 or Full wave ground plane for Railroad

Status
Not open for further replies.

walterb

Member
Premium Subscriber
Joined
May 15, 2008
Messages
82
Hi,

I bought a Radio Shack 102” Steel whip Antenna (# 21-903). I'm wanting to use it to create a full wave or 5/8 wave ground plane antenna mounted on my house roof for receiving railroad traffic (161Mhz) on my scanner. From what I’ve read I need to weld/attach four ¼ wave radio ground planes to the base at ~90 degrees each. They can be at a right straight out or angled down or both.

The question(s) I have are these.

Should I tune the 102” whip to a full wave for 161Mhz to ~ 73 inches. Or should I trim for 5/8 wave ~43 inches, or should I just leave it as long as possible (102").



I already have a Scantenna which is great, but I want a little bit more out of the railroad band if possible.

If you see any flaws in the plan, please let me know. Thanks
 

prcguy

Member
Joined
Jun 30, 2006
Messages
8,100
Location
So Cal - Richardson, TX - Tewksbury, MA
A full wavelength vertical will have degraded performance at the horizon and will receive best at about 45degrees up and down. It will also be difficult to match to 50 ohms. A 5/8 wavelength vertical needs some special attention with matching and a Google search will get you going on that. Unless you have some equipment to tune or analyze a project antenna like this it may work worse than a simple 1/4 wave ground plane that can be made in a few minutes with an SO-239 and some scrap house wire.
prcguy
 

walterb

Member
Premium Subscriber
Joined
May 15, 2008
Messages
82
prcguy said:
A full wavelength vertical will have degraded performance at the horizon and will receive best at about 45degrees up and down. It will also be difficult to match to 50 ohms. A 5/8 wavelength vertical needs some special attention with matching and a Google search will get you going on that. Unless you have some equipment to tune or analyze a project antenna like this it may work worse than a simple 1/4 wave ground plane that can be made in a few minutes with an SO-239 and some scrap house wire.
prcguy
thats not encouraging, however I wanted the truth.. :)

I was planning on feeding it with RG6 Quad shield.

The only thing I have to test it with is to compare it with with the Scantenna and a Marine Band Yagi I have on the roof.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
8,100
Location
So Cal - Richardson, TX - Tewksbury, MA
Making antennas is fun and rewarding and I would encourage anyone to give it a whirl, but without some basic equipment, an exact set of plans or an antenna mentor you can easily produce a lump of coal to stick on your roof. If you want a really good (I mean really really good) antenna for RR and don't mind something 20ft tall, I might make up some plans for an easy to make 4 bay dipole array. It will probably cover most of the VHF hi band with about 6dBD gain omni directional or if you only need to receive a 180 degree or less corridor it can be set up for about 9dBD gain. I'm thinking of using 3/4" PVC pipe and T's to hold the dipole elements away from the mast and you would need to purchase some type F T connectors and a 20ft pipe or fence post and make up some critical lengths of RG6 with F connectors. All I need is some time to make one and publish the dimensions....
prcguy
walterb said:
thats not encouraging, however I wanted the truth.. :)

I was planning on feeding it with RG6 Quad shield.

The only thing I have to test it with is to compare it with with the Scantenna and a Marine Band Yagi I have on the roof.
 

walterb

Member
Premium Subscriber
Joined
May 15, 2008
Messages
82
prcguy said:
Making antennas is fun and rewarding and I would encourage anyone to give it a whirl, but without some basic equipment, an exact set of plans or an antenna mentor you can easily produce a lump of coal to stick on your roof. If you want a really good (I mean really really good) antenna for RR and don't mind something 20ft tall, I might make up some plans for an easy to make 4 bay dipole array. It will probably cover most of the VHF hi band with about 6dBD gain omni directional or if you only need to receive a 180 degree or less corridor it can be set up for about 9dBD gain. I'm thinking of using 3/4" PVC pipe and T's to hold the dipole elements away from the mast and you would need to purchase some type F T connectors and a 20ft pipe or fence post and make up some critical lengths of RG6 with F connectors. All I need is some time to make one and publish the dimensions....
prcguy

Hi,

that sounds wonderful. Many railfans would love something like this. The tracks run in a north / south direction from my house but I'm sure others would want a 360 degree antenna. please let me know how I can help.

thanks
 

kb2vxa

Completely Banned for the Greater Good
Banned
Joined
Mar 22, 2005
Messages
6,126
Location
Point Pleasant Beach, N.J.
If you want a really hot poop rail band antenna New Jersey Transit Rail Operations uses the Cushcraft Ringo Ranger line and the top dog happens to be the ARX2B. I used one for 2M ham and it received very well on the VHF railroad frequencies as well although it wasn't properly tuned for the RR band. Naturally the NJT techs followed the tuning chart and you can too; enjoy a high gain omni directional antenna rugged enough to stand up to anything nature can throw at it short of a tornado.

Tuning isn't TOO critical so don't worry if 161MHz (center of the band) is a bit off the chart; it has a 4MHz 2:1 VSWR curve so it'll cover the entire band and then some. Just feed it with LMR400 or equivalent coax and it'll feed your scanner strong signals. Everybody's got to eat you know.

One last thought, an antenna analyzer comes in handy for fine tuning the ring for a 50 ohm match giving a minimum SWR reading but it's not all that critical for receive. If you really want top performance you just might find a local ham down at the club willing to tune it for you. Don't be shy, if you want the very best you have to work at it just a little.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
8,100
Location
So Cal - Richardson, TX - Tewksbury, MA
The 4 bay dipole array is hotter (steamier?) poop by at least 3dB in omni mode. The Cushcraft ARX2B is supposed to be three 5/8 wave elements in phase, but its really two in phase with a set of radials to suppress feed line radiation. The two 5/8 elements are not at an optimum spacing due to mechanical constraints where the exposed dipole arrays are able to place the elements just right for maximum gain. In reality the ARX2B is probably in the 3Bd gain category but they are popular and tune up ok.
prcguy

kb2vxa said:
If you want a really hot poop rail band antenna New Jersey Transit Rail Operations uses the Cushcraft Ringo Ranger line and the top dog happens to be the ARX2B. I used one for 2M ham and it received very well on the VHF railroad frequencies as well although it wasn't properly tuned for the RR band. Naturally the NJT techs followed the tuning chart and you can too; enjoy a high gain omni directional antenna rugged enough to stand up to anything nature can throw at it short of a tornado.

Tuning isn't TOO critical so don't worry if 161MHz (center of the band) is a bit off the chart; it has a 4MHz 2:1 VSWR curve so it'll cover the entire band and then some. Just feed it with LMR400 or equivalent coax and it'll feed your scanner strong signals. Everybody's got to eat you know.

One last thought, an antenna analyzer comes in handy for fine tuning the ring for a 50 ohm match giving a minimum SWR reading but it's not all that critical for receive. If you really want top performance you just might find a local ham down at the club willing to tune it for you. Don't be shy, if you want the very best you have to work at it just a little.
 

walterb

Member
Premium Subscriber
Joined
May 15, 2008
Messages
82
prcguy said:
The 4 bay dipole array is hotter (steamier?) poop by at least 3dB in omni mode. The Cushcraft ARX2B is supposed to be three 5/8 wave elements in phase, but its really two in phase with a set of radials to suppress feed line radiation. The two 5/8 elements are not at an optimum spacing due to mechanical constraints where the exposed dipole arrays are able to place the elements just right for maximum gain. In reality the ARX2B is probably in the 3Bd gain category but they are popular and tune up ok.
prcguy
I want the hottest poop for the price.. :)

FYI, I picked up some T connectors, and coax yesterday.
 

walterb

Member
Premium Subscriber
Joined
May 15, 2008
Messages
82
prcguy said:
The 4 bay dipole array is hotter (steamier?) poop by at least 3dB in omni mode. The Cushcraft ARX2B is supposed to be three 5/8 wave elements in phase, but its really two in phase with a set of radials to suppress feed line radiation. The two 5/8 elements are not at an optimum spacing due to mechanical constraints where the exposed dipole arrays are able to place the elements just right for maximum gain. In reality the ARX2B is probably in the 3Bd gain category but they are popular and tune up ok.
prcguy
I've done some reading on these and they sound great. If you don't have time to draw up some plans, can you direct me to some online plans, book, etc.?

thanks
 

Mikey54

Member
Joined
Nov 2, 2007
Messages
19
Location
Ontario Canada
The ARRL Antenna Handbook has good plans to build a 4-bay dipole. This book should probably be in your local library, but I would think after seeing it, that you would want to buy your own copy anyway. Lots of good plans, projects, as well as the theory to explain why it works.

Mike VE3MKY
 

walterb

Member
Premium Subscriber
Joined
May 15, 2008
Messages
82
Mikey54 said:
The ARRL Antenna Handbook has good plans to build a 4-bay dipole. This book should probably be in your local library, but I would think after seeing it, that you would want to buy your own copy anyway. Lots of good plans, projects, as well as the theory to explain why it works.

Mike VE3MKY
thanks Mike!
 

prcguy

Member
Joined
Jun 30, 2006
Messages
8,100
Location
So Cal - Richardson, TX - Tewksbury, MA
Most of the plans I've seen have some complicated mechanical requirements or require tuning and test equipment. My goal would be plans that can be easily duplicated with common parts and I need to make one first. Hopefully I'll get to Home Depot today and pick up the hardware to put one together.
prcguy
walterb said:
I've done some reading on these and they sound great. If you don't have time to draw up some plans, can you direct me to some online plans, book, etc.?

thanks
 

walterb

Member
Premium Subscriber
Joined
May 15, 2008
Messages
82
prcguy said:
Most of the plans I've seen have some complicated mechanical requirements or require tuning and test equipment. My goal would be plans that can be easily duplicated with common parts and I need to make one first. Hopefully I'll get to Home Depot today and pick up the hardware to put one together.
prcguy
You have got me very interested in this type of Antenna. I assume their is some physics reason why they are stacked and not build in a circle (thinking hula hoop) or diagonally or some other weird configuration?

I also assume there is some reason they are always the same wave length, and not mixed (i.e, 1/4 wave and 5/8 wave) in the same array?

thanks
 

prcguy

Member
Joined
Jun 30, 2006
Messages
8,100
Location
So Cal - Richardson, TX - Tewksbury, MA
Each dipole is basically a 2 element beam with the mast as the reflector. When you combine antennas you can get up to 3dB gain every time you double the elements but the spacing is critical so they combine in phase to reach the maximum gain. Stacking 4 dipoles at the right spacing will give you 6dB gain omni over a single dipole. If you look at the construction of stacked collinear arrays like the fiberglass Sationmaster, the 1/2 wave elements are right on top of each other and it takes almost twice as many to get the same gain as an exposed dipole array. A 5/8 radiator over a ground plane is a known good performer but dipoles made 5/8 long each leg may have a strange pattern with lobes, I would have to run that on EZNEC and that takes more time than I have at the moment.
prcguy
walterb said:
You have got me very interested in this type of Antenna. I assume their is some physics reason why they are stacked and not build in a circle (thinking hula hoop) or diagonally or some other weird configuration?

I also assume there is some reason they are always the same wave length, and not mixed (i.e, 1/4 wave and 5/8 wave) in the same array?

thanks
 

prcguy

Member
Joined
Jun 30, 2006
Messages
8,100
Location
So Cal - Richardson, TX - Tewksbury, MA
Well, here we go with another antenna project, this one is a 4 element VHF dipole array. I went to Home Depot and bought a bunch of stuff to make an antenna I don't really need, but it should be a good project for others. So far I made the individual dipoles and tuned them to about 150MHz, which will cover below 2m to high 150MHz. In the pic you can see one of the dipoles mounted to a scrap mast for tuning and the method of coax attachment. The picture is distorted from a wide angle lens, the elements are actually very straight. There is also a commercial aluminum version from a Maxrad dipole array for comparison. The project antenna elements are made from 3/4" copper tubing and pounded into 1/2" PVC "T"s. You have to warm the PVC slightly otherwise it might split. The mast attachment end is a 1" to 1/2" reducing "T" that I cut in half and this is a good mate to 1 1/4" TV masting or similar fence post material. The PVC reducer will hose clamp to the mast. All I need to do now is make the phasing harness from RG6 and I can write up all the details and dimensions. Total labor so far was about 1 1/2hrs. The original post wanted a RR antenna and this one will end up as a 2m ham/MURS/Marine antenna for a friend but should also cover RR ok. It can be scaled up to 155MHz easily, which will cover RR better and not loose too much on the low end. More to follow maybe next weekend.
prcguy
 
Last edited:

walterb

Member
Premium Subscriber
Joined
May 15, 2008
Messages
82
prcguy said:
Well, here we go with another antenna project, this one is a 4 element VHF dipole array. I went to Home Depot and bought a bunch of stuff to make an antenna I don't really need, but it should be a good project for others. So far I made the individual dipoles and tuned them to about 150MHz, which will cover below 2m to high 150MHz. In the pic you can see one of the dipoles mounted to a scrap mast for tuning and the method of coax attachment. The picture is distorted from a wide angle lens, the elements are actually very straight. There is also a commercial aluminum version from a Maxrad dipole array for comparison. The project antenna elements are made from 3/4" copper tubing and pounded into 1/2" PVC "T"s. You have to warm the PVC slightly otherwise it might split. The mast attachment end is a 1" to 1/2" reducing "T" that I cut in half and this is a good mate to 1 1/4" TV masting or similar fence post material. The PVC reducer will hose clamp to the mast. All I need to do now is make the phasing harness from RG6 and I can write up all the details and dimensions. Total labor so far was about 1 1/2hrs. The original post wanted a RR antenna and this one will end up as a 2m ham/MURS/Marine antenna for a friend but should also cover RR ok. It can be scaled up to 155MHz easily, which will cover RR better and not loose too much on the low end. More to follow maybe next weekend.
prcguy
Very cool. Just wondering, why use 3/4" pipe instead of 1/2?

thanks
 

walterb

Member
Premium Subscriber
Joined
May 15, 2008
Messages
82
N8IAA said:
Greater bandwidth on receive and transmit.
Larry
Hi, When I hear bandwidth, I think in terms of download speed. Do you mean it will have a wider range of frequencies, or it will be more sensitive or both?

Thanks.
 

Mikey54

Member
Joined
Nov 2, 2007
Messages
19
Location
Ontario Canada
walterb said:
Hi, When I hear bandwidth, I think in terms of download speed. Do you mean it will have a wider range of frequencies, or it will be more sensitive or both?

Thanks.
it will be resonant on a wider range of frequencies.

Mike VE3MKY
 

prcguy

Member
Joined
Jun 30, 2006
Messages
8,100
Location
So Cal - Richardson, TX - Tewksbury, MA
My goal for this exercise is to have an antenna thats works good and is easy to duplicate. The 3/4" copper pipe just happens to fit inside 1/2" PVC fittings (ok, a force fit) and thats less hardware to deal with. Like others mentioned, it helps the antenna cover a wider bandwidth and the entire antenna should have about 20MHz BW or more.
prcguy
walterb said:
Very cool. Just wondering, why use 3/4" pipe instead of 1/2?

thanks
 
Status
Not open for further replies.
Top