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 Post subject: AM flat loop antenna
PostPosted: Feb Thu 28, 2019 2:12 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 419
Location: Loveland,CO
I have a flat loop ant. in progress. The wood cross members are done and its set up for 34turns of #26 wire with 25" between the furthest points of the 'X' frame. (with room at the end for the pick up turns ). Is 34 turns a bit much , should I lose a few ? there is about 1/4" between turns . I don't have a vari cap for tuning yet.

Thanks much , Dave


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 Post subject: Re: AM flat loop antenna
PostPosted: Feb Thu 28, 2019 2:29 pm 
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I'll bet that a lot of these are trial and error. Are you using an inductance calculator? (I don't know if the typical formulas work well for large rectangular configurations).

If nothing else, a reduction in L can be compensated by adding some C. But---isn't the sensitivity also related to the number of turns?

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 Post subject: Re: AM flat loop antenna
PostPosted: Feb Thu 28, 2019 3:59 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 90
Location: Texas, U.S.A.
Dave,
1. I'm not sure what is meant by "flat loop antenna".
2. Will the turns lie on the surface of an (imaginary) tube of square cross section? Given the diagonal of 25 inches, the tube would be around 17.8 inches on a side. Is this what is being constructed?
3. Is the inductance of the antenna intended to resonate with the tuning capacitor?
4. What is the desired frequency range to be received with the antenna?

5. Using this inductance calculator: http://electronbunker.ca/eb/InductanceCalcRc.html the inductance of a square loop of 34 turns spaced 1/4 inch and all lying on the surface of a tube as described in #2 would be around 706 microhenrys.

Eric,
WB5HDF


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 Post subject: Re: AM flat loop antenna
PostPosted: Feb Thu 28, 2019 5:33 pm 
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Location: Loveland,CO
infzqi wrote:
Dave,
1. I'm not sure what is meant by "flat loop antenna".
2. Will the turns lie on the surface of an (imaginary) tube of square cross section? Given the diagonal of 25 inches, the tube would be around 17.8 inches on a side. Is this what is being constructed?
3. Is the inductance of the antenna intended to resonate with the tuning capacitor?
4. What is the desired frequency range to be received with the antenna?

5. Using this inductance calculator: http://electronbunker.ca/eb/InductanceCalcRc.html the inductance of a square loop of 34 turns spaced 1/4 inch and all lying on the surface of a tube as described in #2 would be around 706 microhenrys.

Eric,
WB5HDF

Sorry, by 'flat' I meant like a spider web. It is meant to cover the normal AM band . My thought was I could tune around what ever amount of turns I had using the tuning cap.

I was just wondering if the ant. could get to a point of more turns won't really help performance . Dave


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 Post subject: Re: AM flat loop antenna
PostPosted: Feb Thu 28, 2019 5:56 pm 
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Joined: May Tue 30, 2006 4:46 pm
Posts: 9993
Location: Santa Rosa, CA
If you want to make a loop for the BC band, Dave N2DS, is a pro: http://makearadio.com/loops/loops2.php

His pages show several loops and how to build them.

You won't like a loop with ~700 uH inductance. More in the 200 to 400 range will be better and will work with reasonably available variable caps.

Rich

PS: I have built loops using "hula hoops" as forms. Also 23 inch quilting frames,available cheaply from fabric stores.

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 Post subject: Re: AM flat loop antenna
PostPosted: Feb Thu 28, 2019 7:58 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 90
Location: Texas, U.S.A.
...
Nice radio there Rich. Thanks for mentioning use of a quilting hoop as form.

Dave,

1. Yes, increasing the number of turns on the loop will increase the voltage output of the loop for a given field intensity. The effective height of the loop increases with the number of turns. Output voltage of loop equals effective height of loop multiplied by field intensity - see E. E. Zepler page 162: https://archive.org/stream/Zepler_1943_ ... 1/mode/2up

2. Increasing the number of turns will increase the inductance and capacitance of the loop which will set a maximum resonant frequency with a given variable tuning capacitor. Using 706 microhenrys and guessing 50 picofarads of loop self capacitance and a tuning capacitor minimum capacitance of 20 pF, the resonant freq. would be 716.29 kHz. So yes, 34 turns could be too much to permit tuning to a higher frequency than that.

3. Mr. Robert Weaver has published work on this subject: http://electronbunker.ca/eb/Loop_Antennas.html

WB5HDF


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 Post subject: Re: AM flat loop antenna
PostPosted: Feb Thu 28, 2019 11:23 pm 
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34 turns with that size diameter frame sounds like entirely too many, based on my experience. Half that would be more in the ballpark, I would think. Here's a thread I did on my own BCB tunable loop; viewtopic.php?f=12&t=241661&p=2024254#p2024254

If you scroll down to my second post, you'll see the frequency range with given numbers of turns.

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 Post subject: Re: AM flat loop antenna
PostPosted: Feb Thu 28, 2019 11:46 pm 
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I built this one many years ago. It is rather big.

-Steve

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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Fri 01, 2019 1:44 am 
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^ And notice only 11 turns. What are the height and width dimensions?

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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Fri 01, 2019 3:13 am 
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Location: Saskatoon
Yes, 34 turns seems a bit high. Based on the information given, I get a value of either 460 µH or 750 µH depending on whether the 0.25" pitch refers to the true spacing between the wires or the spacing of the pins along the cross arm. There's a factor of √2 difference between the two dimensions, and that will affect the inductance value significantly. Either way, you should be looking for an inductance value of around 250 µH to be compatible with a standard variable capacitor (assuming that you're tuning the standard broadcast band).

Also, note that the turns close to the center of a loop antenna contribute much less to the overall inductance than the outer turns, because the contribution is proportional to the turn's diameter. They also contribute less to signal pickup because the effective aperture of the smaller turn is also less.


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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Fri 01, 2019 3:19 am 
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Location: Loveland,CO
fifties wrote:
34 turns with that size diameter frame sounds like entirely too many, based on my experience. Half that would be more in the ballpark, I would think. Here's a thread I did on my own BCB tunable loop; http://www.antiqueradios.com/forums/vie ... 4#p2024254

If you scroll down to my second post, you'll see the frequency range with given numbers of turns.



I will cut down to 16 turns, had not wound it yet , but there are 34 brass tacks on each arm., I can just pull out the extras.

As far as the tuning cap, I have a chassis from a 50's AM FM console , could salvage that part .


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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Fri 01, 2019 4:27 pm 
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BobWeaver wrote:
Yes, 34 turns seems a bit high. Based on the information given, I get a value of either 460 µH or 750 µH depending on whether the 0.25" pitch refers to the true spacing between the wires or the spacing of the pins along the cross arm. There's a factor of √2 difference between the two dimensions, and that will affect the inductance value significantly. Either way, you should be looking for an inductance value of around 250 µH to be compatible with a standard variable capacitor (assuming that you're tuning the standard broadcast band).

Also, note that the turns close to the center of a loop antenna contribute much less to the overall inductance than the outer turns, because the contribution is proportional to the turn's diameter. They also contribute less to signal pickup because the effective aperture of the smaller turn is also less.


The one I built so long ago I can't remember uses a varicap out of a transistor radio. I just remember it's quite effective. Each of the two cross pieces is 27.75" long. Mine is not wound flat. On the end of each cross piece is a 6.75" spreader mounted perpendicular to the cross piece. There are 18 windings the cover the entire BCB. To visualize it, if you lay it down flat on the floor, it's like winding 18 turns around a square box 6.75" high.

Bob, do you have any insight which is better, or how they differ, i.e. flat pancake windings vs large solenoid wound like mine?


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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Fri 01, 2019 5:36 pm 
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fifties wrote:
^ And notice only 11 turns. What are the height and width dimensions?


I'll get those tonight. It is rather big. As I recall it is about 38 inches wide.

Sitting on an average size console radio, it almost touches the ceiling.

-Steve

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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Sat 02, 2019 2:27 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2312
Location: Saskatoon
Macrohenry wrote:
The one I built so long ago I can't remember uses a varicap out of a transistor radio. I just remember it's quite effective. Each of the two cross pieces is 27.75" long. Mine is not wound flat. On the end of each cross piece is a 6.75" spreader mounted perpendicular to the cross piece. There are 18 windings the cover the entire BCB. To visualize it, if you lay it down flat on the floor, it's like winding 18 turns around a square box 6.75" high.

Bob, do you have any insight which is better, or how they differ, i.e. flat pancake windings vs large solenoid wound like mine?


As long as the windings are not extremely long in the case of a box loop, or extremely deep in the case of a flat loop, then for the same mean loop diameter and wire spacing, there should be no difference. However, the mean diameter of a flat loop will be less than its outside diameter, while the box type loop's mean diameter will be the same as its outside diameter. So, for the same outside diameter, turns count and wire spacing, the box loop will have higher inductance, and larger effective aperture, but not by very much. If you have a separate pickup loop on the antenna, the type of construction may affect the best way to mount that.

I've seen comments elsewhere claiming that there is a difference in directionality of 90°, but this is completely false. Directionality is the same.

A while back, I built a 24" diameter hexagonal flat loop, mainly because I thought the flat loop looked nicer. The winding depth is only 2 inches. So, the performance would be about the same as a 23" box loop. Not enough difference to worry about.


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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Sat 02, 2019 7:57 pm 
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Location: Wayside, NJ Monmouth
I made this to go with a Franklin design Regen. It's 30" tall over all, arm is 22", and the 16 Turns are spaced 3/8". It needs a cap that is 110 - 1100 uf to tune the entire BCB. Been working on the wooden chassis for this now. But with al the weird weather lately, progress has been very slow.


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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Sat 02, 2019 8:20 pm 
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Location: Dayton Ohio
OK, I measured its size, the cross piece, and loop corner to corner is 40 inches.

11 turns spaced about 1/2 inch apart.

-Steve


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loop-antenna.jpg
loop-antenna.jpg [ 55.55 KiB | Viewed 1498 times ]

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-Zenith
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-Pre-War FM
Consoles and floor models, the bigger, the better!
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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Sat 02, 2019 8:38 pm 
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OK, we are getting some interesting information AFA number of turns vs size.

In my case, 23" diameter = 12 turns.

In azenithnut's case, 40" square = 11 turns.

With N2LXM's square loop, if the actual loop size is 22" = 16 turns. His wiring gets progressively smaller toward the center, which might account for the increase in turns vs my almost identical size, all turns of which are an equal wire length.

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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Sat 02, 2019 10:09 pm 
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Location: Wayside, NJ Monmouth
I started mine after reading through this web page "http://www.earmark.net/gesr/loop/ ". Then I took a piece of thin plywood, laid the pattern out on it using nails. Wound the loop and took a measurement. Added turns till I got where I needed to be. The fun part was building the cross frame, drilling the holes and cutting a grove in the long member for the wires. Still I think it turned out OK.


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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Sat 02, 2019 10:43 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 419
Location: Loveland,CO
N2LXM wrote:
I started mine after reading through this web page "http://www.earmark.net/gesr/loop/ ". Then I took a piece of thin plywood, laid the pattern out on it using nails. Wound the loop and took a measurement. Added turns till I got where I needed to be. The fun part was building the cross frame, drilling the holes and cutting a grove in the long member for the wires. Still I think it turned out OK.



I have tried to post a couple pics here, either it says to big or to small ???

Maybe I could email to a member for posting ? ? Thanks


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 Post subject: Re: AM flat loop antenna
PostPosted: Mar Sat 02, 2019 11:52 pm 
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Location: Dayton Ohio
Ive been using a transistor BC band variable cap with the two sections tied together. Maybe 600 PF fully closed.
It tunes below the broadcast band at the low end, and easily reaches the top of the band at the high end.

-Steve

_________________
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-Zenith
-Sparton
-Pre-War FM
Consoles and floor models, the bigger, the better!


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