RX only loop antenna

[AD0AE]

Hi All-

I have been thinking about building an RX only loop antenna for indoor listening at my apartment.  I have done some background research about this and it sounds like (in general) they are useful for helping to reduce noise and can be aimed to assist in reception.  

I understand that loop antennas are basically LC circuits.  I have a bunch of #10 solid wire that I can use, along with a length of RG -8U if I wanted to make a shielded loop(is there a benefit to a shielded loop...?).  For the #10 wire, I used some formula I found online at various sources and Joe Carr's antenna book to estimate the inductance of a 2 turn, 24-27 inch square loop (6-7 inches on a side).  I also have a 350 pf variable capacitor and a 56 pf variable capacitor I figure I can link in parallel, using the 350 pf as a course tuner and the 56 pf as a fine tune.  I would optimally like to receive 40-20 m (although 80 m might be nice as well...)

Here are my questions...
1. I notice a lot of loops at low frequencies, 160 - 80 m, and VLF frequencies.  It seems to me that because this is a resonant circuit, shouldn't you be able to build a loop for any frequency?  Am I not understanding something?  Is it a design issue?  Or some limiting factor?  I know that to get strong nulls, one should make the loop less than 0.1 times the wavelength (I have seen 0.17 as well).

2.  What is a good way to couple into the loop?  I have seen one design that uses CLC type tuner to couple.  I have seen more than a few that use coupling loops of one turn.  I built a transmitting loop and the "rule" was to make the coupling loop 1/5 the diameter of the main loop... does such a rule still apply here?   I would like to couple to 50 ohm coax.  

3. I am also wondering, is the input of the loop antenna suppose to be balanced a certain way?  And if so, how?  I just want to make sure I understand this.

4.  Are there any other design issues I should be aware of?

Any help would be appreciated!  I just figure if I get some wood or PVC, I should be able to make something that would work!

Thank you all

73s,
Steve
AD0AE (formerly kb9pzm)

[N3OX]

Here are my questions...
1. I notice a lot of loops at low frequencies, 160 - 80 m, and VLF frequencies.  It seems to me that because this is a resonant circuit, shouldn't you be able to build a loop for any frequency?  Am I not understanding something?  Is it a design issue?  Or some limiting factor?  I know that to get strong nulls, one should make the loop less than 0.1 times the wavelength (I have seen 0.17 as well).

No it's mostly just that as frequency rises people tend to favor large, efficient, self resonant antennas even for reception.  The idea of using separate receiving antennas is generally confined to low frequencies because the impracticality of building directional arrays that are also good for transmitting. 

There's no reason why you can't use a loop on any frequency.  I have a 1.5 foot diameter loop that will tune from 7-50 MHz (with a fixed cap switched in for the low end) that I use for noise direction finding... works fine.

2.  What is a good way to couple into the loop?  I have seen one design that uses CLC type tuner to couple.  I have seen more than a few that use coupling loops of one turn.  I built a transmitting loop and the "rule" was to make the coupling loop 1/5 the diameter of the main loop... does such a rule still apply here?   I would like to couple to 50 ohm coax.  

I would strongly recommend a coupling loop myself... I wouldn't use anything with a direct metallic connection to the main loop because you will get better common-mode current rejection if the coax isn't connected to the antenna directly.  The loop size won't be critical for reception; 1/5 is probably still fine.  A reception-only loop can be lossier than a transmit loop with no significant penalty, which will sometimes mean the coupling loop to get a 50 ohm match would be a different size... but an exact 50 ohm match is not really necessary, so I wouldn't sweat it too much. 

3. I am also wondering, is the input of the loop antenna suppose to be balanced a certain way?  And if so, how?  I just want to make sure I understand this.

Inductive coupling can be seen as a really excellent current balun, ensuring that the currents on either side of the "feedpoint" of the main loop are equal.  Of course it does this by not CONNECTING to the main loop at all, so there's no "third leg" to divert current away from the main loop except for a little tiny capacitance between the coupling loop and the main  loop.

So there's not really any problem in simply connecting the coax to the coupling loop.  The "shielded" coupling loop that some people use enforces even more symmetry in a way that may be somewhat useful, but I think the lion's share of the effective transition from unbalanced coax to a balanced loop is simply had by not connecting the coax directly to the main loop.

[WB6BYU]

I agree with Dan.  Loops are most common on the lower bands where a full-sized antenna
is inconvenient, but there is no reason that they can't be used on any frequency.  (Though
at VHF you can get much better performance with other hand-held antennas.)

The limiting factor is the self-resonant frequency of the loop, which takes into account the
self-capacitance between the windings.  You can't resonate a loop conveniently above the
self-resonant frequency.  This coil calculator gives you that information:

http://hamwaves.com/antennas/inductance.html

I have a loop with 3 turns about 8" diameter that tunes 6 - 15 MHz or so with a mica
trimmer.   The coupling loop is about half the diameter of the tuned loop - I did use a
shield loop, but I'm not sure it actually makes a lot of difference.  In this case I used
#12 solid wire and just drilled 3 holes in one end of a 1x2 and 4 some distance further
down, when threaded the wire through the holes to make the loop.  The wire is stiff
enough that I don't need a crossbar.

The diameter of the coupling loop to the main loop diameter for a 50 ohm match depends
on the number of turns in the main loop.  The 1/5 rule is for a single turn loop.  1/2 seemed
to work fairly well for a 3-turn loop.  I also make 80m loops with 4 turns of wire inside a
piece of plastic tubing or hose that is bent into a circle:  in that case I make the coupling
loop the same size and run it through the same tubing, even though it may not give a
prefect match to 50 ohms.  This design has higher self-capacitance, however, because
the wires are close together inside the tubing, and a 6" loop won't go much above 40m.

For operation on the higher bands, I'd leave off the big variable capacitor and just use the
smaller one.

Loops don't pick up as much signal as a larger antenna:  for hand-held 80m receivers I usually
incorporate a cascode amplifier stage using a pair of bipolar transistors.  FETs are often used
with high impedance loops.  In this case the amplifier is designed for 50  ohm output and the
input is designed to match the loop, which need not be 50 ohms.  (The last such 80m loop antenna
I built was only about 4 1/2" diameter.  It - and the receiver it is attached to - appeared in the
latest issue of CQ magazine.)

[WA3SKN]

A receive loop provides a "null" and is useful in tuning out an interfering station.  Expect a 30 dB loss compared to a wire antenna (but your noise will be lower too!)
This will work good for the lower frequencies... 160, 80, 60, and 40 meters.  When you start to get to higher frequencies, a directive beam type antenna that can be used for both transmit and recv makes more sense.
For "receive only", matching is a refinement that is not required to enjoy the loop.
You will find that the open wood or PVC crossed types are affected by nearby metal objects, while the "shielded loop" is not... but the shielded loop is more difficult to build properly.
Give it a try.  It is a cheap project that is easy to build and can be fun to use!
73s.

-Mike.

[K4SAV]

You got some really good info from N3OX and WB6BYU.  Here is another loop calculator you can play with.
www.aa5tb.com/aa5tb_loop_v1.22a.xls

If you build the loop from wire, the gain will be low and if you use it on the high bands a preamp could help.  On the lows bands the gain might be good enough because the atmospheric noise is very high there.  You can make that decision after you build it, and see what you can hear.

The parameter that makes a small loop have low noise is its ability to decrease the gain in two directions (in opposite directions).  It has a null at very low elevation angles in two directions.  That means if you have a single local noise source you might be able to reduce its level by placing it in the null.  If there is more than one noise source you won't be able to get both of them (unless they happen to be 180 degrees from each other.  Don't expect to reduce the atmospheric noise because that usually comes from all directions and a large part of it comes from high elevation angles, where the loop has no null.  If the signal you are interested in happens to come from the same direction as the noise source, you won't get any benefit from the loop.

As for the common belief that the loop can reject noise because it is magnetic, W8JI explained that in this link:
http://www.w8ji.com/magnetic_receiving_loops.htm

Inside an apartment is a very bad place to place a loop because there are usually lots of noise sources being radiated thru the power lines and those are all around you.  If that happens to be the primary noise source, you won't be able to null it because it will be coming from multiple directions. 

Jerry, K4SAV

[WB6BYU]

Inside an apartment is a very bad place to place a loop because there are usually lots of noise sources being radiated thru the power lines and those are all around you. 

Actually it is worse than that - on 80m I find that the loop couples to the electric wiring
acting like a larger antenna.  Signals (and noise) peak when the loop is rotated for maximum
coupling, and dip when rotated for a minimum, regardless of the direction from which the
signal is actually coming from.  Basically your loop antenna is acting like a small coupling
loop to a larger antenna.

We see the same issue with a mobile DF loop when parked under telephone / power / cable TV
wires (and even when the cables are underground, which causes some puzzling results!)