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 Post subject: Problems with Highly Rectangular Loops ?
PostPosted: May Mon 06, 2019 7:22 pm 
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Joined: Jul Thu 12, 2007 9:36 am
Posts: 567
Location: Boston,Ma
Hey All,

I would like to build an inductively coupled loop for my bedside AM b'cast ferrite receiver (s). I have built a few loops/ BIG litz DX crystal sets in the past, so I am pretty good on what /how to use re materials.

My question is: the loop designs out there pretty much show symmetrical designs ie round , or hex or square loops with all sides of the same length. I've seen pictures of some 1920's designs that are VERY rectangular, but were they good performers or just "space savers "?

I would like the biggest bang for my buck re capture area. But since it is to be on the beside table space is tight . More capture area would equate to having to go tall- REALLY tall- ie 1 or maybe 1.5 ft X as much as 4 ft or 5 ft tall.

Some loop sites profess that shape doesn't really matter ( they say increasing SIZE is the key), but I am really "pushing the envelope" too much at 1 or 1.5 ft X 4 or 5 ft ?

I have already built a smaller solid wire loop that is "fairly" rectangular. It seems to perform "very",( if not "extremely") sharp on a high Q var cap .So I guess one can "stretch" things , at least "somewhat".

I have seen radio TANK coil "guidelines" that state in order to maintain good Q, that maybe up to 2 units of diameter should be used for every 1 unit of length . IF this theory translates over to LOOP coils "tall" is better ( but maybe only to a POINT ?).

So anyone have any ideas of the impact on the receive pattern of a greatly "exaggerated" rectangular loop. Is the null as sharp and do we run into any undesired lobes ?

Do you think one would actually HEAR the difference or is it just "somewhat " away from an "optimal" design but still maybe "very good" ?

Obviously I want it as sharp as possible for DX but am not sure of any degree of compromise that may be needed re size ratio.

My shop time is VERY limited these days, so I prefer of sort to get it "right" the first time, rather than by multiple trial and error "rebuilds".

FWIW, I will be using 660/46 litz ,with turns spaced at around 1/4 ", 90 deg wrapped around 1/2" dia styrene tubes running between two rectangular wooden frames.

Var cap(s) will be ceramic insulated multi gang types wired "wiper less" to avoid wiper loss from var cap forks. So the Q of the wire and var cap is "near" optimum, I just don't want to go "backwards" by pressing my dimensional luck too much.


ANY help is greatly appreciated.

Krystallo / N1NQC


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 Post subject: Re: Problems with Highly Rectangular Loops ?
PostPosted: May Mon 06, 2019 7:47 pm 
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Joined: Oct Thu 04, 2018 2:11 pm
Posts: 106
Location: Suburban Chicago
If I remember the theory correctly it is the the area of the loop and the number of turns that count. Loops are directional so you probably want to be able to rotate them easily to get the station you want and if you are so lucky to null out an interfering station. A tall, thin rectangular loop can be rotated in less space than a square or round loop of equal area. I would imagine that is what drives the preference for rectangular loops.

The dimensions of the loop sides are critical when you are trying to build a tuned loop as you might for amateur radio work at higher frequencies where it is feasible to make the loop dimensions large enough. But I think it is just the area that matters on AM broadcast loops which are very much smaller than the operating wavelength.


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 Post subject: Re: Problems with Highly Rectangular Loops ?
PostPosted: May Mon 06, 2019 7:57 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 32010
Location: SoCal, 91387
Did you want a tuned loop? I made one from a 23" quilting hoop and a rotating gimbal. The performance is equal to the 240' random length antenna I have on the back acre.


Attachments:
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4Ab.jpg [ 162.58 KiB | Viewed 397 times ]

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 Post subject: Re: Problems with Highly Rectangular Loops ?
PostPosted: May Mon 06, 2019 8:53 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2284
Location: Saskatoon
As mentioned, it's the area enclosed by the loop that determines how much signal is picked up. For a given amount of wire, a circle encloses the largest area. For a 4 sided loop, then a square gives the largest area.

Inductance doesn't contribute to sensitivity, but it's important to know what it is so you can select an appropriate variable capacitor in order to tune the loop.

Rectangular loop inductance calculator:
http://electronbunker.ca/eb/InductanceCalcRc.html


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 Post subject: Re: Problems with Highly Rectangular Loops ?
PostPosted: May Thu 16, 2019 8:06 pm 
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Joined: Jul Thu 12, 2007 9:36 am
Posts: 567
Location: Boston,Ma
Hey All.I finally decided on my design:660/46 on a frame that can be adjusted to less rectangular if needed. Thanks for the help.K


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 Post subject: Re: Problems with Highly Rectangular Loops ?
PostPosted: May Thu 16, 2019 8:53 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 32010
Location: SoCal, 91387
Please post a scan of it when it's finished.

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