Antenna phenomena?

cassidy1190 · #1 (**permalink**) 10-13-2009, 03:47 PM

I am puzzled by what is going on with my wilson 800MHz yagi. It was picking up alright where i had it mounted in my bedroom, but of course i wanted to see if i could get it to perform any better. I decided to move it up into the attic. For a moment it did not pick up anything. I thought this was very strange, but what i thought was even stranger was that when i decieded to bring it downstairs again, positioning myself behind the antenna, the control channel of the TRS i monitor went off clear as a bell! When i moved away from the antenna...silence. In short, as wierd as this sounds, it seems after a few tests that while in the attic the antenna only works when I am standing behind it. What in the world is going on here?

fineshot1 · #2 (**permalink**) 10-13-2009, 04:08 PM

Its a pretty simple concept - your body is acting like a reflector. This is not uncommon.

jim202 · #3 (**permalink**) 10-13-2009, 04:17 PM

You might want to check out the cable connections on the coax cable. The other thought is
that your pointing the yagi in the wrong direction or you have it pointing into a null.

To do a quick check of the coax, disconnect the antenna and then short the center pin to the
shield in the attic. Then go to the radio end and use an ohm meter and see if you see the
short. It won't be zero ohms, but should be very low resistance on the order of less than 10
ohms. Then go remove the short and measure it again. You should see infinity for an open cable.

If you see a high resistance with the cable shorted the cable connectors have a problem. If you
see any resistance with the attic end open, you have a connector or cable problem.

Jim

Quote:

Originally Posted by cassidy1190

I am puzzled by what is going on with my wilson 800MHz yagi. It was picking up alright where i had it mounted in my bedroom, but of course i wanted to see if i could get it to perform any better. I decided to move it up into the attic. For a moment it did not pick up anything. I thought this was very strange, but what i thought was even stranger was that when i decieded to bring it downstairs again, positioning myself behind the antenna, the control channel of the TRS i monitor went off clear as a bell! When i moved away from the antenna...silence. In short, as wierd as this sounds, it seems after a few tests that while in the attic the antenna only works when I am standing behind it. What in the world is going on here?

cassidy1190 · #4 (**permalink**) 10-13-2009, 04:30 PM

If the reflector scenario is true, what can i put behid the antenna permanently that will produce a signal like that?! It really comes in great. I do not have an ohm meter, but i did try other cables all with the same result. I know its definitely pointed in the right direction too, It cant be in the opposite since i know that the state police repeater is not in the ocean (opposite direction, i live in a coastal town).

prcguy · #5 (**permalink**) 10-14-2009, 12:08 AM

Since its a yagi there is nothing you could add or do as a reflector that would provide any usable benefit except making it the feed in front of a large parabolic reflector.
prcguy

Quote:

Originally Posted by cassidy1190

If the reflector scenario is true, what can i put behid the antenna permanently that will produce a signal like that?! It really comes in great. I do not have an ohm meter, but i did try other cables all with the same result. I know its definitely pointed in the right direction too, It cant be in the opposite since i know that the state police repeater is not in the ocean (opposite direction, i live in a coastal town).

methusaleh · #6 (**permalink**) 10-14-2009, 04:20 AM

I would also venture to guess that you are using a longer coax run to get it up to the attic, which of course is going to give you quite a bit of loss at those frequencies. Perhaps the shortest coax run possible would improve things as well.

cassidy1190 · #7 (**permalink**) 10-14-2009, 01:39 PM

I have my radio set up in a 2nd floor bedroom directly beneath where the antenna is located with only 10ft of LMR-400 between them.

I don’t know guys I tried everything but I just put it back in the bedroom again where it works fine for now. I guess the motto here is, "If it ain't broke don't fix it." Still can't figure out why it was acting funny in the attic though...maybe it has something to do with the octopus of air ducting tubes running all over the attic. I'm sure they are made with some type of aluminum.

MPSCS · #8 (**permalink**) 10-14-2009, 02:55 PM

800 MHz is funny like that. I can usually do pretty good with a stock antenna attached directly to the scanner. However, if I move it a couple inches on my desk, it can make all the difference in the world! Sometimes metallic objects in your home can reflect, or direct signals. Metal duct work for heating & cooling, copper water pipes, electrical circuits, file cabinets, stove, refrigerator, etc.

Just put the antenna back where it worked well with you standing behind it. Find yourself a new girlfriend, bring her home, sit her on a stool behind your antenna and ask her to tell you all about herself. That should work for at least a year or better!

cassidy1190 · #9 (**permalink**) 10-14-2009, 05:08 PM

HAHAHA, she already agreed to the task. Thank you so much as always guys.

ihcollector · #10 (**permalink**) 10-14-2009, 05:28 PM

Wirelessly posted (BlackBerry8310/4.5.0.110 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/-1 UP.Link/6.3.1.20.06.3.1.20.0)

Not sure from your posting how your yagi is setup, but if you don't you might want to put a piece of conduit or metal pipe in the antenna mounting clamp. This will allow the antenna to operate in the proper directional pattern, kind of like what your body did

cassidy1190 · #11 (**permalink**) 10-14-2009, 05:39 PM

so what your saying is mount it to a metal pole? I might have to try that, mine is monted on a sturdy piece of wood.

ihcollector · #12 (**permalink**) 10-14-2009, 06:28 PM

Wirelessly posted (BlackBerry8310/4.5.0.110 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/-1 UP.Link/6.3.1.20.06.3.1.20.0)

Keep in mind, the antenna is designed to be attatched to a metal mast. This is the 'reflector'. This makes the antenna directional. The reflector pipe needs to be at least 2inches longer than the longest elements so if the antenna is 6inches tall top to bottom, then a minimum 10 inch pipe should be used with 2inches above and 2inches below. I think you'll be suprised with the results.

cassidy1190 · #13 (**permalink**) 10-14-2009, 06:47 PM

But isn't the reflector already on the antenna? Its the last and largest element closest to the mast from what i understand. Im really eager to try this, i just have to find the mast...

ihcollector · #14 (**permalink**) 10-14-2009, 07:19 PM

Wirelessly posted (BlackBerry8310/4.5.0.110 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/-1 UP.Link/6.3.1.20.06.3.1.20.0)

The mast is the reflector. The largest element helps establish the gain and directivity along with the other elements (as I remember from my RF engineering class). From practical experience, a yagi without the metal reflector mast/pipe is just shy of being a omni directional antenna with little to no gain. I hope this helps.

cassidy1190 · #15 (**permalink**) 10-14-2009, 11:17 PM

my only question is how will the mast work as a reflector if it is not directily behind the antenna? The clamp is on the left side of the yagi, so that would put the mast about an inch and a half to the left of the elements on the antenna.

ihcollector · #16 (**permalink**) 10-14-2009, 11:31 PM

Wirelessly posted (BlackBerry8310/4.5.0.110 Profile/MIDP-2.0 Configuration/CLDC-1.1 VendorID/-1 UP.Link/6.3.1.20.0)

That's all figured into the design and op specs by the mfgr.

cassidy1190 · #17 (**permalink**) 10-15-2009, 10:15 AM

Wow i never had any idea that something like a mast could improve or even have an affect on the antenna. Im going to run out and get one soon. Thank you for all the information!