Ground Plane question

[BCasto]

I am a scanner enthusiast near Raleigh NC. Most of my listening is from digital trunked systems in the 700Mhs and 800Mhz range. I mainly use antennas on the back of the scanner and they normally do OK. However there are some sites farther away I want to receive.

I have been experimenting with different antennas and the best luck so far is from a TRAM 1089 BNC antenna https://www.amazon.com/1089-BNC-Min...&qid=1523467433&sr=8-1&keywords=tram+1089+bnc

I get the best reception when the antenna is sitting on a metal ground plane. I have read the size of the ground plane should be sized to 1/4 wave. Unless my math is bad, I calculated the ground plane should be between 3.5 in and 4.0 in for 700-800 Mhz.

My question to you antenna experts is this - if the ground plane is much larger than 4.0 inches, will reception be adversely impacted? Also, should the ground plan be round (not square)? I have a 24 inch square piece of metal and want some advice before I start cutting it.

Thanks much.

 

[nd5y]

Ground planes should be a minimum 1/4 wave radius around the antenna. Being bigger won't hurt and probably won't help either.

 

[mmckenna]

My question to you antenna experts is this - if the ground plane is much larger than 4.0 inches, will reception be adversely impacted? Also, should the ground plan be round (not square)? I have a 24 inch square piece of metal and want some advice before I start cutting it.

No, not an issue. Think of a permanent antenna on top of a police car, fire truck, etc. The vehicle roof is a fixed size. As stated above, it ideally needs to be 1/4 wavelength at minimum, but larger will not be a problem.

Adding the ground plane under your antenna will improve things. I'd add that you need to make sure that you seal the outside connections well if you are going to mount that outside.

For what you are doing, a piece of sheet metal, pizza sheet, cookie sheet, etc. will work just fine. Your calculations of 3.5-4 inches is correct. You would not need to cut the metal, though. If you can safely install it as is, that would be fine. I would also help UHF and VHF coverage.

 

[lmrtek]

The 1\4 wave ground radial mistake never seems to get corrected.
In fact ground radials must be at LEAST 10% longer than 1\4 wavelength..
...................
A larger ground plane won't make it work any better but most mistakenly think the ground radials can only be a 1\4 wavelength at the frequency the radiator is cut for.
...........
Antenna companies are interested in making maximum profit so they simply use the least amount if material they can get away with.

 

[prcguy]

Its a common misconception that ground "radials" must be 5 or 10% longer than a 1/4 wavelength. In fact they should be exactly 1/4 wavelength if they are actual radials suspended in air and not on the ground (earth). You will find mention of the 5% length in antenna books from the 50s and 60s, but modern antenna engineering will go for a "resonant" 1/4 wavelength.

The reason is when they are 1/4 wavelength long the RF makes a 1/2 wavelength round trip from the feedpoint to the end of the radial then back to the feedpoint, mimicking the impedance of the feedpoint. When you have two or more opposing radials of equal length you will have equal currents flowing in opposite directions which cancel radiation in the radials leaving only the vertical portion of the ground plane to radiate.

There are some GP style antennas that have extreme downward angles on the radials making almost a coaxial dipole and you will find some of those radials a little longer to facilitate better matching with a little more capacitance between the vertical element and radials which will lower the impedance.

A solid sheet ground plane is a different animal which is rarely resonant, so it follows slightly different rules. Radials on the ground (earth) used mostly for HF will be detuned by the earth and also incur losses usually not found on VHF/UHF antennas high in the air.

What I might do in the OPs case is use the full 24" square plate and maybe plant a tri band Larsen or similar in the future.
prcguy

The 1\4 wave ground radial mistake never seems to get corrected.
In fact ground radials must be at LEAST 10% longer than 1\4 wavelength..
...................
A larger ground plane won't make it work any better but most mistakenly think the ground radials can only be a 1\4 wavelength at the frequency the radiator is cut for.
...........
Antenna companies are interested in making maximum profit so they simply use the least amount if material they can get away with.

 

[rufust]

Would putting a set of 2 different length radial sets be a plus or minus

 

[prcguy]

It can help when the bands are not harmonically related or the round trip length is not a multiple of 1/2 wavelength. For a VHF/UHF combo antenna, radials cut for 155MHz will also be fine for 465MHz because they are 3/4 wavelength long making a 1.5 wavelength or three 1/2 wavelength round trip path.

Radials cut for 150Mhz would not work out well for 850MHz or 220MHz, etc, so an extra tuned set for those bands would probably be beneficial. A flat plate of metal about 3ft across would be fine for everything 155Mhz and up, etc.
prcguy

Would putting a set of 2 different length radial sets be a plus or minus

 

[BCasto]

Thanks for all the good responses. Would any of this matter if the antenna was inside my house (2nd floor). Does the RF signal travel better through a glass window vs. a brick wall? I don't know how else to explain why signal strength varies widely when moving the antenna around in the room.

 

[nd5y]

Would any of this matter if the antenna was inside my house (2nd floor).

Probably not.

Does the RF signal travel better through a glass window vs. a brick wall?

It depends on the glass or brick material.

I don't know how else to explain why signal strength varies widely when moving the antenna around in the room.

That's normal. It happens outdoors too. Being in a building probably has little effect. It is more noticeable at 700 - 900 MHz because of the shorter wavelength there are shorter distances between signal peaks and nulls. It gets more noticeable the weaker the signal is.
https://en.wikipedia.org/wiki/Fade_(radio)
http://www.radio-electronics.com/info/propagation/multipath/multipath-fading.php
http://www.radio-electronics.com/info/propagation/path-loss/rf-signal-loss-tutorial.php

 

[mmckenna]

Thanks for all the good responses. Would any of this matter if the antenna was inside my house (2nd floor). Does the RF signal travel better through a glass window vs. a brick wall? I don't know how else to explain why signal strength varies widely when moving the antenna around in the room.

All materials will have some amount of RF attenuation.

Glass can have coatings to reduce heat loss/penetration, especially newer windows. Those coatings can greatly attenuate signals.
Brick walls can have rebar, conduit, wiring, pipes, etc. in them. That can all impact the ability for signals to pass through it.

Ideally you do want the antenna outside and up as high as you can get it. If that is not an option, put it close to the window. Try opening/closing the window while listening to a steady signal (NOAA weather radio) and see if it has impact.

 

[Mikejo]

Stupid question…

I made a 2 meter J-Pole and I have it suspended from the ceiling inside my apartment. I use it for a scanner only, would placing a ground plane underneath it improve its performance?

 

[prcguy]

A J-pole is a ground independent end fed half wave and would probably not benefit from a ground plane. It has a radiation pattern similar to a center fed dipole but with a little distortion from the junk at the bottom. Most J-poles have a common mode current problem where the coax is not decoupled very well and the outside of the coax will radiate some.
prcguy

Stupid question…

I made a 2 meter J-Pole and I have it suspended from the ceiling inside my apartment. I use it for a scanner only, would placing a ground plane underneath it improve its performance?

 

[daddyjohn]

A J-pole is a ground independent end fed half wave and would probably not benefit from a ground plane. It has a radiation pattern similar to a center fed dipole but with a little distortion from the junk at the bottom. Most J-poles have a common mode current problem where the coax is not decoupled very well and the outside of the coax will radiate some.
prcguy

Does the coax need to be decoupled if it is for receiving only?

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[prcguy]

If you snap on several #43 mix ferrite beads near the antenna it will usually decouple it enough for most purposes. When you do this the radiation pattern will be more predictable and it may reduce a little interference that can ride on the outside of the coax if the coax runs close to switching power supplies, computers, routers, etc. If you can fit two or three turns of coax through one ferrite bead it will usually work better than several beads in a string.

Does the coax need to be decoupled if it is for receiving only?

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