Quarter-wave antenna w/o ground plane?

[jkowtko]

Okay EEs (I'm a Mech E), tell me this --

For one of our receivers in the rack running in the 650-680mHz range, I pulled the quarter-wave whip antennas off the back and moved them out of the rack using a pair of 6 foot RG-58 cables. I mounted the antennas on a luan panel suspended about 6-7 off the ground, spaced several feet apart, and with a great line of sight of the stage. My reception improved a bit. According to theory, it should have degraded due to loss of ground plane. What's happening?

These systems are AKG WMS450 ... Band 1. The whip antenna is the black rubber-coated one and it fatter towards the BNC connector. I don't know if it has a helical winding in it or not -- can't find any documentation. I do know that in contrast, my Band 7 receivers (500-530mHz) have slightly longer, simple chrome post antennas, which I assume are still quarter-wave but lower frequency.

Anyway, all the docs I can find state that the quarter-wave antenna needs a ground plane in order to pick up the signal properly, and that the receiver chassis provides that ground plane.

I should have eliminated that ground plane when I moved the antennas out of the rack. A luan panel should not give me any RF alterations, so the antenna should more-or-less be on it's own out there.

Any idea why it's still working? My last class covering anything related to radio waves was unfortunately freshman physics. The Yahama SR Handbook doesn't even cover wireless, and any info I can on-line find says that my experiment shouldn't really be working. Here's another web site to reference:

Antenna FAQ

If you can explain what's going on, i.e. why I'm receiving the same or better reception than when the whips were in the rack, I'd appreciate it.

Thanks. John

 

[bishopthomas]

I pulled the quarter-wave whip antennas off the back and moved them out of the rack...

I'm not an EE, but I think this is your answer. Antennas like to "see" the transmitters. You provided line of sight. I'm not exactly sure what you mean by "ground plane" unless you're saying that you think the antennas should be at the same horizontal position as the transmitter (which is incorrect, RF does not travel in 2 dimensions).

 

[SHARYNF]

On lower end Wireless units Oscillator interference from the receivers being mounted close together

Oscillator interference is a major factor. vertical and horizontal separation works best for the widest area of pickup

Sharyn

 

[mbenonis]

The ground plane essentially makes the 1/4-wave antenna look like a dipole (due to the theory of reflections, the ground plane creates an image of the antenna below). That said, pretty much anything will act as an antenna. It's just a matter of how well it works.

 

[jkowtko]

Mike, I know what the ground plane is supposed to do ... from the docs though it implies that you must be able to pick up a half-wave with an antenna in order for it to work, and therefore implying that if you don't have a ground plane, the 1/4 wave antenna will not work. Is that not the case? (obviously it isn't since my contraption is working ... I'd just like to know why).

Thanks. John

 

[FMEng]

Mike, I know what the ground plane is supposed to do ... from the docs though it implies that you must be able to pick up a half-wave with an antenna in order for it to work, and therefore implying that if you don't have a ground plane, the 1/4 wave antenna will not work. Is that not the case? (obviously it isn't since my contraption is working ... I'd just like to know why).

Thanks. John

Rarely do antennas just not work. It's usually more a degree of how well or badly they work. A clip lead wire shoved in the connector works too, just not too well. Chances are, your antennas are not picking up as much signal as they would otherwise.

If your set up works without noise or drop outs, then no need to change anything. But if the system runs into trouble, it could perform better if the wood were replaced with sheet metal. Aluminum plate is a favorite because it is easy to work with and does not rust. Large, blank rack panels are easy to come by. Use a Greenlee metal punch to make the holes for the antennas.

 

[gcpsoundlight]

(Please note this is coming from a Licenced Amateur Radio operator before you badger me)

Depending on what you did connection wise, you may well have extended the ground plane.

Is the material that you mounted the connectors on electrically conductive? if it is, good.

Did you mound the BNC's with rivets, screws or something else? if there is a good electrical connection, then that is good.

For those who don't know, a ground plane is the reference to the antenna's voltage, kinda like a neutral in mains. In most instances, the rack box will be made of metal and act as a ground plane. If you used coax with shield connections at both ends, the panel you mounted the bnc's to was metal, and the bnc's are in some way connected to the metal, then you have just extended the ground plane.

having said that, an antenna will work without a ground plane, however it's performance will be reduced.

I would now like to put a wise old saying on the table: "If it ain't broke, don't fix it."

Hey, sometimes shoving a bit of wire in the backs works. you are not going to bust the reciever by connecting a different antenna, or changing it's postition. At worst, it just won't pick up the signal.

Hope this helps.

 

[Chris15]

(Please note this is coming from a Licenced Amateur Radio operator before you badger me)

Depending on what you did connection wise, you may well have extended the ground plane.

Is the material that you mounted the connectors on electrically conductive? if it is, good.

Did you mound the BNC's with rivets, screws or something else? if there is a good electrical connection, then that is good.

For those who don't know, a ground plane is the reference to the antenna's voltage, kinda like a neutral in mains. In most instances, the rack box will be made of metal and act as a ground plane. If you used coax with shield connections at both ends, the panel you mounted the bnc's to was metal, and the bnc's are in some way connected to the metal, then you have just extended the ground plane.

having said that, an antenna will work without a ground plane, however it's performance will be reduced.

I would now like to put a wise old saying on the table: "If it ain't broke, don't fix it."

Hey, sometimes shoving a bit of wire in the backs works. you are not going to bust the reciever by connecting a different antenna, or changing it's postition. At worst, it just won't pick up the signal.

Hope this helps.

It was noted that they board holding these is plywood, which is not conductive. At UHF frequencies, the connector footprint does not represent any significant fraction of a wavelength and so isn't going to provide a useful ground plane. At microwave frequencies this is not as true...

If a non tuned antenna didn't work then people you replaced dead car antennas with a bent coathanger wouldn't get radio... It just don't work as well as an antenna of correct length.

Note all this changes somewhat significantly when you are transmitting and want your gear to behave and not complain...

 

[jkowtko]

I'm at the point where the one pair of antenna I've extended out from the rack gets slightly better reception than the others. It's not perfect but it is noticably better.

It sounds like if I replace the luan panel with a metal one, and ground it to the power circuit that I am using for the rack equipment, I should get another bump in signal strength.

Question though -- does the antenna need to be positioned perpendicular to the ground plane for most effective pickup? Half of my antennas have a fixed 90 degree BNC connector, so as they are currently attached to the mounting panel, the antenna is parallel to and about a half inch from the panel's surface. I'm guessing I would need to reorient the antenna to be perpendicular to the panel ...

 

[Chris15]

and ground it to the power circuit that I am using for the rack equipment

The simpler and more effective option is to ground to the coaxes, chances are the connectors you mounted to the luan will ground the panel when you install them on the metal panel...

 

[jkowtko]

The simpler and more effective option is to ground to the coaxes, chances are the connectors you mounted to the luan will ground the panel when you install them on the metal panel...

You are right -- the pass-thru F-F BNC panel connectors have insulated and non-insulated versions ... I'll just get the non-insulated one

Thanks. John

 

[SHARYNF]

just to clarify what may be obvious, if the antenna is in a vertical position then the ground plane needs to be below the antenna in a horizontal position. SO in essence you would have a metal shelf with the antenna sticking up from it. If on the other hand you have the the antenna horizontal sticking out from a metal panel, then the antenna is not going to receive the signal well, If you have the antenna on a bnc and the antenna rotates so that even though the bnc in horizontal the antenna is vertical and the metal panel is behind the antenna in a vertical parallel position, then you are not going to get the performance. Usually the coax run should be 5 feet or under and Belden 5513 or better, and remember that each BNC/f connection has a signal loss.

Perhaps overly nit picky but .....
Sharyn

 

[gcpsoundlight]

I agree. I should have mentioned that. I guess I have been talking to people who are also radio enthusiests and don't need to say that (if I did, they would give me funny looks!)

 

[SHARYNF]

Gerard
Thanks I have been trying to make sure that in technical exchanges I probably overly describe some of the details since this site is used by a lot of students who may or may not be aware of the details.

Sometimes people think I am being condescending to them when I do this since I know and they know that I know that they are experienced and obviously know these details, but someone just reading the posts might not be that knowledgeable

SO glad you understand

Sharyn

 

[jkowtko]

No, not at all Sharyne ... people from all backgrouns on these forums, and sometimes they just don't have common sense, so it can't hurt to point out the obvious just to make sure it wasn't overlooked I always try to qualify these types of remarks so I don't sound condescending ...

I was planning to respond last night but got caught up in work stuff ... but in poking around a bit on ground plane antenna designs, it looks like "plane" carries a broader meaning:

http://www.arrl.org/tis/info/pdf/ab18-16.pdf

I forget that the huge amateur radio movement in the "old days" (my dad was a radio man, and one of our neighbors growing up had a short-wave antenna above his house, about 20 feet in diameter) brought about a lot of home-built contraptions ... and it's very conceivable that somewhere between a coat hanger and a $600 set of active paddles, I can come up with something for very little cost that will do the job nicely.

Not sure that I want to spend all the time building this though, other than for the pure enjoyment of the hobby and the challenge -- it's probably more cost-effective for me just to buy the expensive hardware and be done with it. But I'm always one for the underdogs ... if you can prove that you don't have to spend the big bucks to get the results ....

 

[SHARYNF]

Those are the designs that use radials. Typically this design is used mounted on a pole so that you can get the line of sight up high, and still have the ground plane effect and not go with a 1/2 of full wave antenna.

Personally in that situation I would have going with a full wave antenna since you are not looking at a very long antenna

It is a fun topic

Sharyn

 

[gcpsoundlight]

Oh boy, we've opened a can of worms here! I agree at 800 meg (Assuming here), a full wave vertical is only about 400mm. So may as well do that.

 

[jkowtko]

Oh boy, we've opened a can of worms here! I agree at 800 meg (Assuming here), a full wave vertical is only about 400mm. So may as well do that.

In my [very light] reading today, one of the sites mentioned that 5/8 wave is more optimal than full wave:

design your own 5/8 wave vertical antenna

There's another site that has a full wave antenna, but it's not straight -- it's looped and allows you to tune it by sliding the connecting points up and down the wire:

2 Meter Full Wave Loop - The Magic Wand Antenna by WB5ISM

I personally like the appearance of the dipoles. And, if I have to have 16 pairs of them mounted on a wall somewhere they won't look too obtrusive and won't require a ground plane:

How to make a Simple Dipole Antenna | nathan.chantrell.net

Then there is the log periodic wide-band selling for $25 ... looks just like the SHure paddle without the black spray paint

400-1000 MHz PCB Log Periodic Antenna - eBay (item 300373276384 end time Mar-31-10 20:17:48 PDT)

And then there's ...

Homebrew antenna shootout

 

[gcpsoundlight]

I wouldn't go for the 2 meter loop, that is intended mainly for Amateur 2 way radio use, and won't tune up properly.

 

[Chris15]

If you are going to bother with building antennas, then you'd be far better off DIYing a distribution system than building 16 or 32 antennas...