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 Loop Antenna
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 Return to top of page · Post #: 1 · Written at 9:06:22 AM on 9 July 2017.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 1445

With my fiddling with the Chieftain it has come to my attention that I dont entirely know how a loop antenna works. So I am asking if any of you have the time could you write a fairly indepth explanation as I am sure I am not the only one thats not real sure. Oh and how to count Valve pins as I have rather embarasingly learnt how to do this .
Cheers Guys.


 
 Return to top of page · Post #: 2 · Written at 9:25:19 AM on 9 July 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3860

The circular valve pins are normally read clockwise from the spigot in Octals, the gap in miniatures looking at the base, after that you look at the data on somewhere like "Franks electron tube pages"or the Philips "Miniwatt Technical data". There are traps like Octals where 2&7 are normally heaters, but there are a few ferals where that is not so: Assume nothing. Even after 50 years of fixing, the data book is my friend

A guy now deceased: Who founded our Radio Club, wrote a book called "Crystal sets & such" there are still a few copies available I believe.
Try Pages-Cobram "pages.cobram.bigpond.com" if not I can possibly round one up, if you're interested.

Despite the title, aerials & coils are critical to Crystal set performance, just as it is with powered radio: These are covered.


 
 Return to top of page · Post #: 3 · Written at 9:40:22 AM on 9 July 2017.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1112

 
 Return to top of page · Post #: 4 · Written at 11:07:52 AM on 9 July 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1345

Yes Carl you now know how to count valve pins. Clockwise from underneath.

ICs are clockwise looking from the TOP. Funny that!

Best way to think of a loop antenna is just as a physically large inductor, tuned with a capacitor to make what is called a parallel tuned circuit - best sensitivity on a particular frequency is set by the number of turns of wire and the variable capacitor.

The bigger the loop is (with a given number of turns of wire), the better it works as an antenna to intercept the electromagnetic radiation put out by the transmitter.

In your radio there will be a trimmer - a 2nd and much smaller variable capacitor - in parallel with the loop and the main tuning variable capacitor. Some old radios actually had this trimmer on the loop itself and accessible through a small hole in the case so it was accessible with the radio fully assembled in its case. Its purpose is to allow the loop antenna or antenna circuit to "track" the local oscillator in the 1R5 mixer more accurately. You should adjust this trimmer on a weak or distant station around 1300kHz. carefully to get maximum volume. Do this and your radio suddenly becomes much more sensitive, picking up lots more stations.

Most radios have a similar trimmer connected across the other half of the tuning capacitor. Its purpose is to allow the dial to line up with its markings at the top end of the dial. It should be checked before adjusting the loop's trimmer.

Oh yes. The loop also forms a DC path to the AGC line or ground for grid 3 on the mixer valve - 1R5 in your case. Without this path the grid floats around all over the place with either no signal, noise or other strange unexpected results.

Hope I am not trying to tell grandma how to suck eggs!


 
 Return to top of page · Post #: 5 · Written at 11:18:17 AM on 9 July 2017.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 1445

Its a very interesting read actually. Smile


 
 Return to top of page · Post #: 6 · Written at 5:37:36 PM on 9 July 2017.
JFB's Gravatar
 JFB
 Location: Milton, NSW
 Member since 27 June 2016
 Member #: 1945
 Postcount: 36

Not sure if I am speaking out of turn here but I think there is a small weeny error in Ian's earlier post, being an old retired computer engineer I can't speak of all ICs but certainly for RTL, DTL, TTL and CMOS IC's they are measured anti-clockwise from the top. Viewed from the top the dot in the top left hand in the encapsulation identifies PIN 1, from there go anti-clockwise. AFAIK this applies to analogue IC's as well.

Fault finding in logic circuits generally is from above the PC board, for radios with chassis, it's from below which may explain the difference.


 
 Return to top of page · Post #: 7 · Written at 7:15:59 PM on 9 July 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1345

Just checking you were paying attention!

Of course you are right. One of my mental inversions!


 
 Return to top of page · Post #: 8 · Written at 9:37:34 PM on 9 July 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3860

It really is a case of being on the ball: You can get caught out. The old UX4 sockets were numbered clockwise looking at "base or wiring side" but you could easily get tricked by the fact that many were top wired & then it goes counter clockwise. Philips "P" base is one that goes the other way.


 
 Return to top of page · Post #: 9 · Written at 10:26:21 AM on 10 July 2017.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 1445

"Oh yes. The loop also forms a DC path to the AGC line or ground for grid 3 on the mixer valve - 1R5 in your case. Without this path the grid floats around all over the place with either no signal, noise or other strange unexpected results."

Aha so this could also account for why I have very little volume as well? Am I Correct.


 
 Return to top of page · Post #: 10 · Written at 4:04:54 PM on 10 July 2017.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1345

Absolutely.

G3 of the mixer (1R5) must have a DC path to ground, or to a suitable negative voltage. The loop antenna provides this path.

It was not unusual with battery valve portables to run the front end "flat out" (no AGC, only the filament-derived bias), so as to get the best performance out of the tiny signals picked up by a small loop antenna - the biggest they could fit in the case.

Some of the best performing loop antennas were wound around the outside of the case, in a wide groove in the timber, and covered with the usual leatherette. I have one such radio in my collection where genuine leather has been used to cover it. It's a very sensitive radio. .


 
 Return to top of page · Post #: 11 · Written at 12:41:28 PM on 14 July 2017.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 730

If you need to replace a loop antenna, or want it bigger to make the radio more sensitive:
I just made an antenna for a radio I installed into an old bookshelf speaker (not a good one, so don't worry). I made it large to capture more signal out of the air. What to do: Get a spool of wire about a hundred feet long. Select wire without solder coating on it. Skin effect would force most of the RF current through the solder, which has a higher resistance than copper, and that will kill the Q. Better yet if you have a supply of silver coated wire, usually done with Teflon wire (Teflon does bad things to copper, so the silver plating). Silver is a better conductor than copper, but as silver is much more expensive, everyone uses copper to wire their houses. Or use litz wire for the antenna if you happen to have some. Anyway, get some corrugated cardboard from that box your latest ebay win came in, and make a coil form. Cut an odd number of notches one every 5 cm or so. You will wind the wire in a basket weave pattern. This basket wave pattern needs the odd number of notches or else it won't work out. That reduces stray interwinding capacitance. You have seen this done in older AA5 antennas. I made one about 38cm by 28cm. It took about 17 turns. But be prepared to add or remove turns or fractions thereof. Smaller size loops will need more turns. Tune the radio the antenna is for to a weak station near the bottom of the band, like 558KHz. Connect the inner wire of your antenna to the "hot" part of the antenna tuning cap, and the outer wire to the AVC "RF ground". Use a high impedance voltmeter to monitor the AVC voltage. Add or remove turns of wire from your antenna to peak the signal strength of that station selected above. A fractional turn is okay. A few solder joints in the antenna won't hurt anything as long as you can't get shorted turns. You can have a few feet of wire between the antenna and AVC line as you're tweaking the antenna. You are actually adjusting the antenna inductance to resonate it to the tuning cap. After you get the peak, tune up to a station around 1500KHz and tweak the antenna trimmer cap. If you did the basket weave pattern, you should be able to get a peak. The radio should be more sensitive now. Check that strong stations are not distorted or causing intermod in the band. Now you're done.


 
 Return to top of page · Post #: 12 · Written at 8:15:07 PM on 15 July 2017.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 3860

Just be very careful as to how the set is laid out. I have seen a few examples where a loop has been allowed to extend into an un-shielded zone (and the odd wire). This oversight has resulted in the loop picking up the internal radiation & using it to destabilise the whole show (oscillation)


 
 Return to top of page · Post #: 13 · Written at 11:39:38 PM on 21 December 2017.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 895

For any DIP IC the dot marking pin 1 is bottom left and count anti-clockwise from there.
PLCC and QFP packages are usually top centre pin 1.


 
 Return to top of page · Post #: 14 · Written at 12:05:39 AM on 22 December 2017.
Art's Gravatar
 Art
 Location: Somewhere, USA
 Member since 22 October 2013
 Member #: 1437
 Postcount: 895

The characterising attribute of a magnetic loop antenna is that they are sensitive to the magnetic component of a radio wave,
rather than the electrical component, and their primary, or a secondary coupling loop is a DC shot circuit.
They aren't as sensitive as a comparable Hertz/Marconi antenna, but have significantly better signal to noise ratio, and noise floor,
so for receiving you can overcome the sensitivity issue with either RF or audio amplification, and still be better off.

The ferrite loop antenna found in the later pocket transistor AM radios was the greatest size/cost payoff in radio history by a mile,
but they are completely useless outside of the MW band, and also useless radiators (for transmission) on any band.

For other bands, or even MW where space allows, air core magnetic loop antennas are frequently used as the receiving antenna
for transceiver stations, while a second antenna of another variety is used as the radiator for transmission, to take the pros from both.

QFH (Helicoidal antennas) used for GPS, and J-Pole antennas used for VHF both fall into the category of magnetic loops and exhibit
the same attributes as any other.


 
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