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I was a member of this group some time ago... somehow I became disconnected but the daily message content continued to be delivered to my e-mail address. While continuing to read the mail, I was struck by a construction note that discouraged the use of variable condensers for tuning purposes.because, as I interpreted it, the result would be weak signals. I decided to experience this phenomenon myself, so I have built a relatively crude loop, based on materials already at hand plus a 30 inch hula hoop. I was startled at how strong the signals are, because I had been led to believe that a loop delivered somewhat weaker signals along with a substantial loss in noise, in other words a better signal-to-noise ratio at the cost of a few S points on a tuning meter. I do have solid-state varactors on hand but I have been trying a selection of two-gang tuning condensers, mostly from surplus stores of around fifty years ago. I concluded that the weak link would be the connection
to the moving vanes (rotors) which is made via a ball-bearing race at one or both ends, and a spring brass or phosphor bronze contact finger resting against the metal frame. With 50 years of oxidation on the contactor, this could lead to vastly diminished performance by introducing resistance or diode action (or both) into the circuit. Therefore, I prepared the end of the contactor for soldering rather than a pressure fit. I rubbed it CAREFULLY (!) with a sand or emery board such as used in manicuring and tinned it with solder without damaging the rest of the structure. One end of the loop would be soldered there. The other point of contact ia where the grounded shaft rotates against the finger. To deal with the oxide there, I gave a couple of puffs of contact cleaner and worked the rotation about 20 times and hoped for the best. I also gave a couple of puffs to the ball race and to the shaft bearing at the back of the condenser. My loop is crude: a single turn of
insulated stranded hook-up wire fastened to the outer edge of the hula hoop with strips of masking tape about every 8 inches. The tuning conderser is mounted at the top of a piece of wood about 64" x 1 1/2 " , the loop mounted directly below with a couple of plastic ties, one end soldered to the contact finger, the other end soldered to the fixed plates (stators). For the pick-up loop, the experience of other people suggested 1/5 the diameter of the main loop, which would put me at 6". I happened to have on hand a circular antenna of the type that used to be supplied with TV sets using UHF tuners.... it has a diameter of 9" but that is what I am using for now. It is connected via RG58/U to the 52 ohm input of my Grundig Sattelit 800 and at the moment (0205 utc) I am listening to a strong signal from Habana, Cuba on 6180 khz. I know I have gone on and on somewhat, but I felt it worthwhile to dispel the notion that old tuning capacitors were no good and, who knows,
it might encourage some hesitant souls to try their luck at constructing a "crude" loop. John MacKay, VE7AFN.

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At 07:16 PM 5/26/2007 -0700, you wrote:

I was a member of this group some time ago... somehow I became disconnected but the daily message content continued to be delivered to my e-mail address. While continuing to read the mail, I was struck by a construction note that discouraged the use of variable condensers for tuning purposes.because, as I interpreted it, the result would be weak signals. I decided to experience this phenomenon myself, so I have built a relatively crude loop, based on materials already at hand plus a 30 inch hula hoop. I was startled at how strong the signals are, because I had been led to believe that a loop delivered somewhat weaker signals along with a substantial loss in noise, in other words a better signal-to-noise ratio at the cost of a few S points on a tuning meter. I do have solid-state varactors on hand but I have been trying a selection of two-gang tuning condensers, mostly from surplus stores of around fifty years ago. I concluded that the weak link would be the connection
to the moving vanes (rotors) which is made via a ball-bearing race at one or both ends, and a spring brass or phosphor bronze contact finger resting against the metal frame. With 50 years of oxidation on the contactor, this could lead to vastly diminished performance by introducing resistance or diode action (or both) into the circuit. Therefore, I prepared the end of the contactor for soldering rather than a pressure fit. I rubbed it CAREFULLY (!) with a sand or emery board such as used in manicuring and tinned it with solder without damaging the rest of the structure. One end of the loop would be soldered there. The other point of contact ia where the grounded shaft rotates against the finger. To deal with the oxide there, I gave a couple of puffs of contact cleaner and worked the rotation about 20 times and hoped for the best. I also gave a couple of puffs to the ball race and to the shaft bearing at the back of the condenser. My loop is crude: a single turn of
insulated stranded hook-up wire fastened to the outer edge of the hula hoop with strips of masking tape about every 8 inches. The tuning conderser is mounted at the top of a piece of wood about 64" x 1 1/2 " , the loop mounted directly below with a couple of plastic ties, one end soldered to the contact finger, the other end soldered to the fixed plates (stators). For the pick-up loop, the experience of other people suggested 1/5 the diameter of the main loop, which would put me at 6". I happened to have on hand a circular antenna of the type that used to be supplied with TV sets using UHF tuners.... it has a diameter of 9" but that is what I am using for now. It is connected via RG58/U to the 52 ohm input of my Grundig Sattelit 800 and at the moment (0205 utc) I am listening to a strong signal from Habana, Cuba on 6180 khz. I know I have gone on and on somewhat, but I felt it worthwhile to dispel the notion that old tuning capacitors were no good and, who knows,
it might encourage some hesitant souls to try their luck at constructing a "crude" loop. John MacKay, VE7AFN.

Hi John,

If you are building a tuned loop for MF or HF receiving purposes the quality of the (air gap) variable capacitor is of minor consequence. In most cases the advantages of a tuned loop (even those using 30 year old variable capacitors with some rust in their joints) is a quantum leap ... and in most cases they are fed into or inductively coupled to modern high gain receivers. The quality of the capacitor becomes significant when the tuned circuit performance is more critical as for example when using a crystal radio receiver, or when feeding power to the loop antenna.

There are some tricks that can be used with old variable capacitors with questionable contact surfaces .... for example if you have a 2 gang (365 pfd per gang) capacitor you can connect them in series and isolate the frame from the circuit. This will give you a maximum of 182 pfd capacity .... but no questionable contact surfaces. If you have a 4 gang 500 pfd capacitor (as someone on this group mentioned) you could hook them into 2 parallel/ series pairs and have a full 500 pfd capacitor with no contact surface to deal with.

The added advantage is that the voltage capability is also doubled. If your dealing with power, then the capacitor problem is not only one of rusty joints but voltage capability .... since voltage across the capacitor gap rises very quickly as power is increased.

A tuned loop (about 3' in diam.) is my favorite receiving antenna when living at the apartment on the 9th flr. I put mine on the balcony and tune it remotely with 12 ft of 300 ohm twin lead fed into a Z-match antenna tuner. In order to rotate it I walk out on to the balcony and give it a twist hi hi . PS ... I have fed power into such a loop and have made several contacts ... but the design was intended for reception only. I am sure if I constructed the loop from larger diameter copper tubing instead of RG-58 coax and heavy duty home brew open wire feed line instead of 300 ohm twin lead, it would make a decent antenna for transmission as well.

Jim, VE3CI

At 12:06 pm ((PDT)) Sun May 27, 2007, Jim Dunstan wrote:

[big snip]
A tuned loop (about 3' in diam.) is my favorite receiving antenna when
living at the apartment on the 9th flr. I put mine on the balcony and tune
it remotely with 12 ft of 300 ohm twin lead fed into a Z-match antenna
tuner.

Does this mean there is no capacitor attached to the loop
and all tuning is done by the ATU? So the resistance of the
feeder is included in the loop resonance?

If so, you could get higher Q and hence more output by
tuning at the loop. OTOH, Q is one of those good things
one can have too much of. ;-)


Regards, LenW
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From Yahoo! Groups Help: ... trim all the irrelevant quoted text
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At 02:34 PM 5/28/2007 +0100, you wrote:

At 12:06 pm ((PDT)) Sun May 27, 2007, Jim Dunstan wrote:

[big snip]
A tuned loop (about 3' in diam.) is my favorite receiving antenna when
living at the apartment on the 9th flr. I put mine on the balcony and tune
it remotely with 12 ft of 300 ohm twin lead fed into a Z-match antenna
tuner.

Does this mean there is no capacitor attached to the loop
and all tuning is done by the ATU? So the resistance of the
feeder is included in the loop resonance?

If so, you could get higher Q and hence more output by
tuning at the loop. OTOH, Q is one of those good things
one can have too much of. ;-)

Regards, Len

Hi Len,

You are correct on all counts. The original purpose of the arrangement was to provide a decent antenna for my SWL activity (Sony 7600 receiver) from 5mhz to 15mhz. The tuner sits inside the window looking out on the balcony. The operation is simple .... when it hits resonance the noise level goes up on the receiver .... that's it!! Compared to the whip antenna the performance is a quantum leap improvement, even if I bring the receiver/whip out on the balcony.

I also built transmitting loops for operation at the apartment, using the more traditional tuning/matching arrangements. They work fine .... when I tuned in to SW station (eg 15mhz ... close to 20M) I could NOT detect appreciable difference in performance listening to SW broadcast stations. Of course, this kind of tuning is not compatible with band hopping.

In order for a loop to be an effective SWL antenna it must able to be mounted remotely and provide a reasonable signal level across a very broad range (at least 5-15 mhz or more). In order to provide a reasonable signal across this broad spectrum the antenna either has to be remotely tuned or be broad banded with amplification at the source. My system is the first ... remotely tuned .... with the distance limitation of approximately 15-20 ft. with balance feed line. If you go beyond that distance the capacitance of the feed line starts to limit tuning range. It is also possible to tune such a loop with coax feed line but the distance is even more limited as coax represents greater capacitance.

Jim, VE3CI

I wish I could find a simple design for a SWL loop antenna
that would cover, at least, say, between 5 and 10 MHz. Just
something simple I could stick up in the yard, maybe. All these
amplifiers, tuning capacitors, etc. Where is a simple loop?

(Crying in my beer this holiday...) //// Richards ////
=================================================================

Jim Dunstan wrote:

I wish I could find a simple design for a SWL loop antenna
that would cover, at least, say, between 5 and 10 MHz. Just
something simple I could stick up in the yard, maybe. All these
amplifiers, tuning capacitors, etc. Where is a simple loop?

Not easily done. Loops are inherently low impedance antennas, and when
you're well below the first resonance they look a lot like an inductor. As
you approach the resonance, they begin to look like a shorted transmission
line. I had to tackle this when I was designing my own active loops, and I
was able to devise a remote tuning and amplifier using the coax as both the
amplifier power and the tuning voltage. It became a 2-part series in QEX.

A 1M diameter loop made with 0.25" copper tubing (first resonance at
27.5MHz) will have an impdance of 0.5 + j110 ohms at 5MHz, 1.0 + j 230 ohms
at 10MHz, 1.5 + j450 ohms at 15MHz, and 25 +j1500 ohms at 25MHz. Not
exactly a linear relationship.

The same antenna can be used at 40M with just a 64:1 impedance
transformer (three 4:1 transformers in series) and a fixed capacitor, but
you will still need some sort of tuning somewhere in order to have it
function well over the entire band. Making the transformers so they will
have low loss is a real pain.

Chris

,----------------------. High Performance Mixers and
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Richards,

Like Jim Dunstan said the loop doesn't need to have a tuning cap. You
can use a simple wire loop with a remote tuner. Google for "Z-match"
and you will find a simple and very capable tuner. If you need to run
anything longer than 20' or so for a feedline you can use 450 Ohm TV
ladderline (if you can find it) or make some ladderline with #20 wire
spaced an inch or so apart. That should be good for upwards of 50'
with very little loss. I have even used runs of 20' of 300 Ohm foam
filled twinlead with good success.

Loop size?? Whatever you can erect!! I have used square loops with
sides as small as 2' and as large as 25'. The Z-match tunes them all!

Right now I'm using my Z-match to tune an indoor loop strung around
two sides of my bedroom and tuned through about 10' of #22 speaker
wire. Yes, it's noisy but it works better than a random wire or dipole
from inside my 2nd floor urban apartment.

73,

'Bear' NH7SR

OK -- yet most Z match tuners seem to be home brew. Does
either my MFJ-956 or MFJ-16010 tuner qualify? Or do I need
something else? IS there an inexpensive commercially made
one that works for cheap? If not cheap, for a lot?

Thanks - I will Google around on this...

Given what you say, I could make a loop on the outside of my deck railing, which would be 20 feet by 5 feet, and all be about ten feet
above ground. From your comment that would be a decent loop, and
tunable in the shack with a z match tuner (assuming I figure out
what one is and how to get or make one....) Hmmmm... I like this
prospect.

/// Richards ///
===========================================================

Michael Hebert wrote:

Richards,

The MFJ-956 and MFJ-16010 tuners are both intended for use with random
length wire antennas... not loops. MFJ does make loop tuners. The
MFJ-933 and -935 but they are not Z-match designs. Those tuners are
meant to be mounted at or _very_ close to the loop.

Using any tuner remotely located from the loop (including the Z-match)
will distort the loop pattern since the feedline becomes part of the
loop. With close-spaced wire like speaker cable there will be greater
signal loss than with wider spaced wire like 300 Ohm twinlead or 450
Ohm ladderline. I follow a very loose rule of thumb that says the loop
circumference should be a minimum of 4 times the length of the
feedline. That seems to ensure that the loop does not simply act like
a short circuit at the end of the feedline. The greater the loop
circumference the more signal capture area it will have... and that's
the name of the game. The tuner is simply used to enable a reasonably
efficient transfer of that energy to the receiver.

The Z-match is not difficult to build. Mine uses one 365pF variable
cap, 1 dual-365pF variable cap, the coil wound on a T80-6 toroid and a
DPDT switch to choose between the two output link windings. I believe
I used the design on G3YCC's web page as a guide. The whole thing is
build into a black plastic enclosure from Radioshack. The caps and
toroid were ordered from Ocean State Electronics.

It's certainly much cheaper to build one of these than to buy an MFJ
tuner... and, IMHO, it's much more flexible in use for receiving. I
have used mine for transmitting also but the caps limit power handling
to a max of about 5 watts.

73,

'Bear' NH7SR