NOAA ground plane collinear antenna

[AronDouglas]

I'm putting together a NWS monitoring station (finally) and I figured I'd build my antenna rather than buy one. I am still green in the world of antennas, so forgive any stupid questions or ideas.

I plan on making a 1/4wave ground plane antenna How to Build an Antenna But as I was reading along I came across collinear antennas, and that got me thinking. Can I have a 162.500Mhz 1/4 wave collinear ground plane antenna, or is such an antenna redundant or not necessary? Typically 1/4 wave antennas have no gain, and in my limited reading I found that collinears have up to 3db gain.

I know for 162.500Mhz NOAA monitoring you dont need a great antenna since they have rather powerful stations everywhere, but I figured I'd have some fun and see what I could build.

 

[davenlr]

If you are planning on trying to get WX from distant locations, rather than just monitoring your close in stations, I would recommend a Quad Loop beam with two or 3 elements, or a 3 or 4 element yagi antenna. The problem with monitoring 1000W NOAA broadcasts, is when the band is open, you often will get two or three or more stations coming in at once. A directional antenna will let you zero in on one at a time, and are fairly easy and compact to build. The downside, you will need an antenna rotor.

If you are going to wind a coil for a collinear, consider the 5/8 wave over 1/4 wave. It will give you lots of gain.

 

[AronDouglas]

Never thought of using a directional antenna, I might do that.

I have 3 NWS within 100 miles of me, so monitoring one directly shouldn't be too hard.

 

[prcguy]

The most gain you will get for a VHF omni is from properly spaced dipoles and there is a good DIY 4-bay dipole array project on RR that works great on 2m through NOAA weather frequencies. Here is a link to the project. I've made a few of these and have known the author for quite some time.
http://forums.radioreference.com/build-your-own-antenna/109144-4-bay-vhf-dipole-array-project.html

You can also make a collinear from lengths of coax soldered together and the center/braid reversed at each connection and here is a typical construction project for that.
Omni-Gain Vertical Collinear Antenna by WA6SVT

The problem with stacked whatever wavelength elements using 1/4 wave/90deg phasing coil or hairpin loop between them is the elements are not optimally spaced to achieve the maximum gain for the number of elements you have stacked together.
prcguy

 

[majoco]

There's a good article here...

http://www.arrl.org/files/file/QST/This Month in QST/January 2014/VirtualRadarJan2013QST.pdf

...on building a colinear for adsb 1080MHz but it may give to some ideas for a lower frequency version. IMHO they don't need a ground plane.

 

[davenlr]

There's a good article here...

http://www.arrl.org/files/file/QST/This Month in QST/January 2014/VirtualRadarJan2013QST.pdf

...on building a colinear for adsb 1080MHz but it may give to some ideas for a lower frequency version. IMHO they don't need a ground plane.

Heck with the antenna, I ordered the stuff to see if I can pick up the aircraft in flight. Never knew about this stuff before. Thanks for the link. For $25, it sounds like a great little project. The antenna is pretty neat to, the way he puts the peices together without having to solder.

 

[AronDouglas]

Thanks guys for the antenna ideas. And thanks prcguy, I had totally forgotten about the 4-bay dipole array. I think I even have that pdf saved somewhere. Only question I have with that antenna is can I sue 1/2" or 3/8" instead of 3/4" copper?

 

[prcguy]

The project is very repeatable with the specific hardware listed and if you change anything be prepared to make adjustments to accommodate. If you have access to an antenna analyzer you can make the elements thinner and tune them for a specific freq but thinner elements will be slightly narrower in band width.

One nice thing about the project is 3/4" copper elements fit nice and snug in the PVC T sections and another size pipe will require some testing to see what fits. You either buy the specific parts, follow the instructions and have a known working antenna, or spend some time experimenting.

I was fortunate to have some hardware on hand that fit together nicely and popped out what you see in the instructions in one afternoon. I also documented everything as it progressed since the exercise was going to end with some sort of instructions for someone (I forgot who) that had asked me how to make one. It was one of those lucky days where my brain was functioning and everything worked fist time.
prcguy

Thanks guys for the antenna ideas. And thanks prcguy, I had totally forgotten about the 4-bay dipole array. I think I even have that pdf saved somewhere. Only question I have with that antenna is can I sue 1/2" or 3/8" instead of 3/4" copper?

 

[davenlr]

prcguy. Isnt that the same antenna, on a commercial scale, that FM radio stations use? Looks the same as the ones on the big transmitting towers.

 

[AronDouglas]

I was looking over again the plans for this antenna How to Build an Antenna, and it says I need 3/16" rod for the ground planes. Is this really important or can I use 3/8" aluminum hollow tubing rods?

 

[davenlr]

Not important if you arent transmitting, and dont want wide bandwidth. If its just for one frequency, it will be fine. A better antenna, with gain over a 1/4 wave would be a 1/2 wave coaxial dipole, which you can just make at the end of your coax, and slide into a 1/2" piece of PVC and mount with two U clamps. Ive built several (just measure the 1/4 wave from the end of the coax, cut around the jacket (but not the shield), and fold the sheild back over the jacket so you have 1/4 wave of sheild folded back over the jacket, and 1/4 wave of center conductor showing at the end. Works with 50 or 75 ohm coax, and is totally weatherproof if sealed inside the PVC pipe.

 

[AronDouglas]

In the link I posted about the ground plane antenna (first post), why does it say to use elements 18 3/16" long? For a frequency of 162.5 (as stated in the link) you should need 17.283". Am I missing something here?

I'm also getting some conflicting information about that flat 6"x6" aluminum plate. I know its meant for a ground plane, but is it rely necessary?

I know this is just a receiving antenna, and that virtually anything would work better than having no antenna. But this is one of those things I'd just like to know (been trying to understand antenna theory

 

[AronDouglas]

Ok, found something REALLY cool to do. I'll do this Help is On the Way for attaching ground planes. Only question remains is how large should I make the aluminum plate (I've seen antennas with no aluminum plate and some with 2-6 inch plates). If make a 4inch ground plate, do I deduct that length from the radial elements? Basically what I'm asking is, where do you measure ground plane radial lengths from? Is it from the versicle element or from the SO-239 mount/aluminum plate mount?

 

[WA0CBW]

You could eliminate the plate and connect the radials directly to the UHF connectors 4 holes. In the design shown the plate is used to mount the radials. The length of the radial is from the UHF connector to the tip of the radial. Reducing the length and drooping of the radials is used to make the antenna a better match to the 50 ohm coax.