Ground - plane not sure ?

[usnasa]

Hello all
I currently have a radio shack mag mount antenna (http://www.radioshack.com/product/index.jsp?productId=2102469)
inside my condo near a window due to no antennas outside , my Question is should I just get a bigger piece of metal or should I add wire radials around a four inch metal plate which my antenna sits on now for better performance. and if so what type of wire should I use and how long should I make it to target 800 mhz frequiences mostly
thanks

 

[Freq_n_Hertz]

Hi usnasa,
A 4" diameter or 4x4" square should suffice for 800 MHz but for lower frequencies you might see an improvement by increasing the size of the ground plane.
For UHF (460 MHz) you should have at least a 6" preferably a 12" and if VHF (155 MHz) is a concern then you should go with at least 18", preferably 36".
Radials made of wire will suffice, regular household copper wire will work well. Heavier gauges are better but anything of sufficient length will do (eg. length more important than diameter for this application)

If improving 800 MHz is your biggest concern then you might consider a different antenna specific to 800 MHz and accept degraded performance on the other bands. Height is also extremely important, the higher the better and can often make larger improvements than antenna design.
Also consider cable losses between the antenna and the radio. It is conceivable for 800 MHz, where cable losses are higher, that a short antenna connected directly to the rear of the radio would perform better than one on a long cable mounted a short distance away.

Hope this helps!

 

[kb2vxa]

For "800MHz frequencies mostly" all you need is the antenna sitting in the center of a 3" diameter circle of metal around it. The largest you would need is 19" for VHF Hi Band, soldering 4 stiff copper wires to whatever you use for a mounting plate making the shape of an X will do it. Radials for Lo Band are too long to be practical indoors unless you want to lay them on the floor and trip over them. The thing about radials is you don't need a whole bunch cut for each band. Cut them for the lowest frequency band you listen to, they work all the way to the highest just as well but not the other way round. If 800 is your band of choice and the others are of little interest just leave it as-is.

 

[usnasa]

so i have my antenna sitting on a four inch metal and i was told it was not enought metal ? so your saying if i place my antenna on a 19' x 19 " piece i would get best results for all around frequiences as well as 800 mhz without the wire radials ?

 

[N_Jay]

For "800MHz frequencies mostly" all you need is the antenna sitting in the center of a 3" diameter circle of metal around it. The largest you would need is 19" for VHF Hi Band, soldering 4 stiff copper wires to whatever you use for a mounting plate making the shape of an X will do it. Radials for Lo Band are too long to be practical indoors unless you want to lay them on the floor and trip over them. The thing about radials is you don't need a whole bunch cut for each band. Cut them for the lowest frequency band you listen to, they work all the way to the highest just as well but not the other way round.

3" RADIUS (not diameter) for 800 MHz,

 

[N_Jay]

so i have my antenna sitting on a four inch metal and i was told it was not enought metal ? so your saying if i place my antenna on a 19' x 19 " piece i would get best results for all around frequiences as well as 800 mhz without the wire radials ?

A 38" x 38" piece for VHF on up.

 

[usnasa]

wow a 38 x 38 piece is big for indoors is not practical i receive all frequiences well except for some 800 mhz so what would help my problem sorry to ask but i keep getting different answers i want to improve 800 mhz

 

[zz0468]

wow a 38 x 38 piece is big for indoors is not practical i receive all frequiences well except for some 800 mhz so what would help my problem sorry to ask but i keep getting different answers i want to improve 800 mhz

Your problem may not be your antenna ground plane. It may be absorption from the walls and trees and things outside. Don't expect any miracle cure for poor reception with an indoor antenna. The fix is to put the antenna outside.

 

[usnasa]

I understand the outside is best but i am unable to do that because of the condo rules but thanks for all who took the time to answer

 

[N_Jay]

wow a 38 x 38 piece is big for indoors is not practical i receive all frequiences well except for some 800 mhz so what would help my problem sorry to ask but i keep getting different answers i want to improve 800 mhz

Just hang some wires off the ground.

 

[crayon]

wow a 38 x 38 piece is big for indoors is not practical ..

Gravity and the second law of thermodynamics sucks too, but that's physics for ya. I suppose you can just think of it as the "man" holding ya down.

:roll:

I too own that same antenna when I first got started scanning and it is a very poor performing antenna. If you want to target 800 MHz then you should get an antenna that is cut to those frequencies.

 

[usnasa]

i can unscrew the top part off of that i wonder it that would help ?

 

[usnasa]

hang wires where and what type of wires thanks

 

[crayon]

i can unscrew the top part off of that i wonder it that would help ?

naw. Do not unscrew it as it all comes apart if you do that.

 

[usnasa]

no the top part the coil comes off with a hex screw thanks for helping me what about the wire thing what type ?

 

[crayon]

no the top part the coil comes off with a hex screw ..

o .. *that* top part. You really need to be more descriptive with your questions, especially since, to be blunt, you do not know what you are doing.



The length of an antenna determines which frequencies it is going to receive on. Given your knowledge/skill level on this subject, you can disregard using wires to construct an antenna. I am not trying to be rude, it is just that there is a point of diminishing returns and if you are really serious about learning, you need to start off with some basics first.

Please consider purchasing an antenna that is designed to receive the frequencies that you are interested in for now whilst you learn.

 

[N_Jay]

Are you sure it is the antenna and not just where you are?

 

[crayon]

Are you sure it is the antenna and not just where you are?

It is a pretty crappy antenna so it could be a combo of both .. FWIW.

 

[Freq_n_Hertz]

usnasa,
Location of the antenna is very critical.
Significant changes in height usually have a bigger impact than antenna design.
It seems as though 800 MHz is your main concern so your ground plane needs to be 6" diameter (3" radius) that KV2VXA suggested. Adding longer radials or larger piece of metalwork won't make a noticeable difference at 800 MHz.
You might try moving the antenna to different locations around the room to find an optimal spot.
Shortening the antenna to 3" might also help 800 MHz but will degrade the other bands. You could try taking the top piece and stick it directly into the base.

Have Fun!

 

[N_Jay]

Start with a simple antenna. Best is a 1/4 wave for the band you want.

If that does not get you a fairly good signal, then you are not going to get much better without getting more height or getting out of the building.