ground for loop antenna 2

[ve1msg]

Hi There,

I'm having a little difficulty with a loop
antenna that I designed and built. The loop
antenna is part of a communication system
that operates at a frequency less than 10MHz.
I wanted to use a loop antenna because I
thought that if you drove such an antenna
from a balanced source, you wouldn't neen
to have a local ground. However I have
experimentally determined that if I use a
wire that is about 3ft long to act as a
parasitic ground that I can improve the
gain of each Tx or Rx link by 14dB! The
ground is added by connecting the 3ft wire
to one or the other (it doesn't matter
which side it is connected to) of the loop
antenna terminal connections. Has anyone
ever experienced this type of phenomena
before? I would very much like to have a
system that didn't require this ground.
Thank you for your help.

Sincerely,
Mark

 

[VEBill]

How big is the loop ? And what is the frequency ?

 

[hacksaw]

sounds like you are using an unshielded loop(?), that kinda' defeats its advantage (as a loop), and your design will act like a coiled antenna depending on the size. in that case grounding is always beneficial.

what type of feed line are you using and how are you achieving a balanced output on your source...

 

[ve1msg]

Hi There,

The antenna consists' five turns in a 10 inch diameter loop. The way I am driving it is with a TTL line driver. I have for bits connected in parallel on each side of the loop. It is configured such that when one four bit group goes "high" the other goes "low". Of course I have put a capacitor in series with my loop so the dc of the line driver doesn't pass through. I am driving the loop directly and at resonance. I thought that by driving a balanced configuration I would avoid the necessity of requiring a ground? Do you have any ideas? Thanks for the help.

Sincerely,
Mark

 

[VEBill]

That little 10-inch loop is rather small for 10MHz. It's also more of a tuned circuit than an antenna. What you've discovered is that the 3-foot dangling wire is radiating more than your tuned circuit.

[NOT REALLY RECOMMENDED] Make the dangling wire about 47-feet long (half-wave end-fed, Hi-Z feed point) and you'll really get out.

In the long term, have you considered the issues that might arise if you continue to bang out square waves on the HF band? Not only is 10MHz reserved for other purposes, your square wave will be all over the HF band (and possibly beyond). It's a relatively low level signal, but sooner or later someone might complain about it. And then, being Canada, somebody would ask you to stop.

 

[hacksaw]

sounds like a great jammer circuit, with harmonics generated all over the place.

loop antennas are tuned systems. you've got a induction coil not an antenna as such. adding wire simply increases the radiation efficiency.

 

[Comcokid]

ve1msg-
I am not particularly familiar with loop type antennas. But, some forms of loop antennas are trimmed by a capacitor at the top of the loop connected between elements.
The fact that you are operating <10MHz, and find a 3-foot wire improving your antenna leads me to suspect an impedance matching issue. Such a short wire for this frequency is going to more likely add capacitance at the feedpoint more than actually act as a radiating element.
-comcokid

 

[BrianR]

A frequency of 10MHz corresponds a wavelength greater than 30m and a 10&quot; loop is miniscule in terms of wavelength and will have a radiation resistance close to zero. As the radiation resistance is the measure of radiation the efficiency of this loop will be very low, probably in the order of < 0.01% so it will have a gain of -40dB or less.

A loop does not need a ground and your 3ft wire will not be acting as one, but it do other things. It is probably just as good a radiator as the loop and so gives an apparent gain (actually a lower loss).

The loop should be resonated at the operating frequency with a parallel capacitance which significantly improves it's efficiency but it's bandwidth will be very small, like < 1KHz. A random piece of wire attached to one end will add capacitance and significantly change the tuning and either improve or reduce performance depending on how closely to the operating frequency the loop was tuned in the first place.

Matching a small loop to a transmitter is difficult. I^2*R loosses are high due to the low impedance and the voltage across the input terminals is very high due to the effect of resonating the loop. It's usually considered too difficult to match to a transmitter and they are not used for that. They are useful as a receiver though with their ability to null out noise and the drop in noise more than makes up for the poor efficiency.

Hope this helps.