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Re: Varicaps vs old fashioned variable capacitors
On 4/30/2014 7:39 AM, n3ikq@... wrote: Put another way, Does the mechanical complexity of a remotely tuned
variable capacitor provide any advantages over varicap diodes with all Varicaps definitely can rectify and create QRM from strong signals, be they AM BCB stations or nearby ham stations or if you are running SO2R. The back to back configuration helps but you can still have rectification. You said you were using a preamp. Preamps also create their own intermod so it can be hard to determine whether any given intermod is due to the preamp or varicaps. This includes built in preamps in radios. The IC706 preamp is bad for this. Rick N6RK
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Re: Varicaps vs old fashioned variable capacitors
Hi all, I'm experimenting with various remotely tuned open air loops and ferrite loopsticks. I have made
a remotely tuned varicap diode assembly and a remotely tuned variable capacitor assembly (using a
stepping motor w/arduino). I intend to experiment with each on my 4x4 open air box loop and soon a
loopstick as well. I am also building up a differential preamp so I will not have to bother with a second
pick up loop. My question is: What is the disadvantage of using varicaps (in pairs) compared with using a
variable capacitor to tune a loop? Do varicaps have any tendency to overload or buckle under strong noise
conditions? Put another way, Does the mechanical complexity of a remotely tuned variable capacitor provide
any advantages over varicap diodes with all other things being equal? According to circuits I have obtained
on the net, I can drive a differential preamp with either setup. A thorough search of the net has not
provided me any data concerning my question. My only conclusion to date is that my stepping motor imparts
significant noise into the antenna when it is operating. This is not actually a bad thing in that it tends
to serve as a tuning aid when using a traditional receiver. Of course when using an SDR with a spectrum
scope, the noise is a distraction because one can see the antenna peak moving along the frequency domain
as it is tuned. Any thoughts welcome!
Varicaps can produce serious IMD products when used for remote antenna tuning, especially in the presence of strong nearby broadcast stations. Variable capacitors will always give far better performance in those circumstances. Varicaps will also degrade the equivalent antenna noise temperature due to finite bulk resistance, whereas variable capacitors have little if any series resistance. I have not gone to the trouble of doing a comparison between the two, I just know from experience about the IMD problems of varicaps. I have recently devised a single variable capacitor approach to tuning a loop antenna, and if I can ever get my act in gear I'll publish it online as I have done with other designs. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE
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Varicaps vs old fashioned variable capacitors
Hi all, I'm experimenting with various remotely tuned open air loops and ferrite loopsticks. I have made a remotely tuned varicap diode assembly and a remotely tuned variable capacitor assembly (using a stepping motor w/arduino). I intend to experiment with each on my 4x4 open air box loop and soon a loopstick as well. I am also building up a differential preamp so I will not have to bother with a second pick up loop. My question is: What is the disadvantage of using varicaps (in pairs) compared with using a variable capacitor to tune a loop?? Do varicaps have any tendency to overload or buckle under strong noise conditions? Put another way, Does the mechanical complexity of a remotely tuned variable capacitor provide any advantages over varicap diodes with all other things being equal? According to circuits I have obtained on the net, I can drive a differential preamp with either setup. A thorough search of the net has not provided me any data concerning my question. My only conclusion to date is that my stepping motor imparts significant noise into the antenna when it is operating. This is not actually a bad thing in that it tends to serve as a tuning aid when using a traditional receiver. Of course when using an SDR with a spectrum scope, the noise is a distraction because one can see the antenna peak moving along the frequency domain as it is tuned. Any thoughts welcome!
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Applicable to receive loops?
Applicable to receive loops?
Discuss...
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Re: Shielded Loop Center Element
What are you saying, that the loop will work better with the center conductor attached, or what.
On Mon, Apr 21, 2014 at 4:32 PM, <gregella53@...> wrote:
?
A loop antenna primarily responds to the magnetic component of a radio wave.? An unshielded loop will also pick up some of the electrical component, and not necessarily in a symmetrical manner.? If you want the deepest possible nulls and a symmetrical pattern, either for direction finding or to null out an unwanted signal, a shielded loop is better.? Also more immune to local noise.
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Re: Shielded Loop Center Element
A loop antenna primarily responds to the magnetic component of a radio wave.? An unshielded loop will also pick up some of the electrical component, and not necessarily in a symmetrical manner.? If you want the deepest possible nulls and a symmetrical pattern, either for direction finding or to null out an unwanted signal, a shielded loop is better.? Also more immune to local noise.
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Re: Shielded Loop Center Element
Garey wrote on April 17.
?
?
Guys:
¡°Wondering what the "advantages / disadvantages" are for a
Shielded Loop vs a non-Shielded Loop for the same Loop? DIAMETER.? I
have a "feeling" that one has a better "Q" than the other - implying one has
better "UNWANTED" signal rejection.
Garey Wittich??? Santa
Monica,CA¡±
?
?
?
Garey,
?
A balanced? 1 turn loop of the some diameter driving into a low z
input amplifier will have similar near E field rejection as a screened
loop.? I did a lot a work with screen and unscreened versions of the
ALA1530 in 1997 and to be honest I didn¡¯t see any advantage with
shielding.???? Having to use a smaller diam. conductor
increases the loss compared to using a conductor of the same diam. as the
screen.?
?
Notwithstanding the above, a screen may be necessary for a tuned or
multi-turn loop if it can¡¯t be balanced.
?
73
?
Andrew Ikin
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Re: Shielded Loop Center Element
Guys:
Wondering what the "advantages / disadvantages" are for a Shielded Loop vs a non-Shielded Loop for the
same Loop ?DIAMETER. ?I have a "feeling" that one has a better "Q" than the other - implying one has
better "UNWANTED" signal rejection.
Some of the advantages are low frequency noise immunity and isolation from nearby objects. I have a section on the noise immunity on my paper on shielded loops in the group files section. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE
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Re: Shielded Loop Center Element
Guys:
Wondering what the "advantages / disadvantages" are for a Shielded Loop vs a non-Shielded Loop for the same Loop ?DIAMETER. ?I have a "feeling" that one has a better "Q" than the other - implying one has better "UNWANTED" signal rejection.
Garey Wittich ? ?Santa Monica,CA
?? ?
On Tuesday, April 15, 2014 5:21 PM, Everett Sharp wrote:
?
Guys,
One of the best things that I have found is PAP pipe ( polyethylene - aluminum- polyethylene). I found some 1" on eBay and have used it to build both a 1 meter loop and a 6' diameter shielded loop. By using number 22 magnet wire as the inner conductor, the capacitance was about 7 pF per ft. I have the 6' loop configured in a m?bius. It is the best working antenna I have covering from daylight to 33 MHz. It problem will work well above 33 MHz, I just don't have a receiver to listen above that. I plan on building a 7.5 ft loop using 1.5" OD 0.30" aluminum tubing in the next 2 months or so.?
The PAP is used primarily in Europe for Gas line feeds and in floor heating. It is very easy to bend.
Everett N8CNP
On Apr 15, 2014, at 6:52 PM, Andy Gardner
< ceo@...> wrote:
?
PVC wouldn't be good for Q. An HDPE jacket would be far superior.
Maybe there's irrigation pipe available with suitable diameters?
On 16/04/2014, at 6:37 AM, Brian wrote:
>
> Chris,
> We used 6mm copper tubing with a bonded PVC jacket
> for instrumentation lines at work, 10mm overall diameter
> so that would fit nicely down 15mm copper water pipe as the loop
> Just a thought.
> 72, Brian, G?NSL.
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Re: Shielded Loop Center Element
Guys,
One of the best things that I have found is PAP pipe ( polyethylene - aluminum- polyethylene). I found some 1" on eBay and have used it to build both a 1 meter loop and a 6' diameter shielded loop. By using number 22 magnet wire as the inner conductor, the capacitance was about 7 pF per ft. I have the 6' loop configured in a m?bius. It is the best working antenna I have covering from daylight to 33 MHz. It problem will work well above 33 MHz, I just don't have a receiver to listen above that. I plan on building a 7.5 ft loop using 1.5" OD 0.30" aluminum tubing in the next 2 months or so.?
The PAP is used primarily in Europe for Gas line feeds and in floor heating. It is very easy to bend.
Everett N8CNP
toggle quoted message
Show quoted text
On Apr 15, 2014, at 6:52 PM, Andy Gardner < ceo@...> wrote:
?
PVC wouldn't be good for Q. An HDPE jacket would be far superior.
Maybe there's irrigation pipe available with suitable diameters?
On 16/04/2014, at 6:37 AM, Brian wrote:
>
> Chris,
> We used 6mm copper tubing with a bonded PVC jacket
> for instrumentation lines at work, 10mm overall diameter
> so that would fit nicely down 15mm copper water pipe as the loop
> Just a thought.
> 72, Brian, G?NSL.
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Re: Shielded Loop Center Element
PVC wouldn't be good for Q. An HDPE jacket would be far superior.
Maybe there's irrigation pipe available with suitable diameters?
toggle quoted message
Show quoted text
On 16/04/2014, at 6:37 AM, Brian wrote:
Chris,
We used 6mm copper tubing with a bonded PVC jacket
for instrumentation lines at work, 10mm overall diameter
so that would fit nicely down 15mm copper water pipe as the loop
Just a thought.
72, Brian, G?NSL.
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Re: Shielded Loop Center Element
My present 10' shielded loop is used only for the BCB and below so much of
this would likely not affect my system as much as if it were to be used on
higher frequencies,
But, this would be a bit difficult to construct, possibly using 1/2"
tubing for the outer element, 5/15" tubing for the inner element, and some
plastic tubing for the insulator. Not sure of the benefit of doing this
would be worth the effort. yes...I agree. I believe Everett constructed a similar high efficiency
shielded element but not sure if he was able to compare it with RG8 or RG11
at low frequencies to measure the changes. In a system limited by
atmospheric noise levels I'm not sure there is much to be gained by
rebuilding my present RG58 shield element. At 10' diameter, anything heavier
would eventually suffer storm failure here on the ocean.
There may be an indirect advantage. With the bulk resistance reduced, the Q of a series-tuned loop would increase, which would result in a narrower tuned bandwidth and consequently lower received noise power. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE
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Re: Shielded Loop Center Element
Chris,
We used 6mm copper tubing with a bonded PVC jacket
for instrumentation lines at work, 10mm overall diameter
so that would fit nicely down 15mm copper water pipe as the loop
Just a thought.
72, Brian, G?NSL.
Brian, That would be an interesting construct for a shielded loop. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE
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Re: Shielded Loop Center Element
Chris,
??????? We used 6mm copper tubing
with a bonded PVC jacket
for instrumentation lines at work, 10mm overall diameter
so that would fit nicely down 15mm copper water pipe as the
loop
Just a thought.
????????????????????????????????????????
72, Brian, G?NSL.
toggle quoted message
Show quoted text
Sent: Tuesday, April 15, 2014 4:14 PM
Subject: Re: [loopantennas] Shielded Loop Center
Element
?
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Re: Shielded Loop Center Element
Thanks for the articles Chris. My present 10' shielded loop is used only for the BCB and below so much of this would likely not affect my system as much as if it were to be used on higher frequencies, But, this would be a bit difficult to construct, possibly using 1/2" tubing for the outer element, 5/15" tubing for the inner element, and some plastic tubing for the insulator. Not sure of the benefit of doing this would be worth the effort. yes...I agree. I believe Everett constructed a similar high efficiency shielded element but not sure if he was able to compare it with RG8 or RG11 at low frequencies to measure the changes. In a system limited by atmospheric noise levels I'm not sure there is much to be gained by rebuilding my present RG58 shield element. At 10' diameter, anything heavier would eventually suffer storm failure here on the ocean. Steve WEB - "The VE7SL Radio Notebook":
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Re: Shielded Loop Center Element
a larger diamer inner element of a shielded loop will have lower bulk
losses, therefore better efficiency. would it be something one could notice (measure) on a 10' loop? My present
10' shielded loop uses RG-58 and going to RG-8 or RG-11 would really
increase the weight. Is it something worth considering? How would the
increased efficiency manifest itself...increased overall system
gain...better S/N ratio or ...??
I sent you a couple of files to illustrate how the loop element diameter and frequency affects the efficiency of a non-shielded loop antenna. The results can be extrapolated to understand the effects of using different diameter materials for a shielded loop. Something that I've been meaning to look into is the overall effrects of the characteristic impedance of the shielded loop element. We often use a small diameter wire for the inner element and sometimes even use 50-ohm coax. But, the terminal impedance of the shielded loop is small, commonly less than 5 ohms. That being the case, the characteristic impedance of the shielded loop element should be the same as the terminal impedance. But, this would be a bit difficult to construct, possibly using 1/2" tubing for the outer element, 5/15" tubing for the inner element, and some plastic tubing for the insulator. Not sure of the benefit of doing this would be worth the effort. Chris When the going gets weird, the weird turn pro - Hunter S. Thompson
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Re: Shielded Loop Center Element
Thank you Chris. a larger diamer inner element of a shielded loop will have lower bulk losses, therefore better efficiency. would it be something one could notice (measure) on a 10' loop? My present 10' shielded loop uses RG-58 and going to RG-8 or RG-11 would really increase the weight. Is it something worth considering? How would the increased efficiency manifest itself...increased overall system gain...better S/N ratio or ...?? Steve WEB - "The VE7SL Radio Notebook":
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Re: Shielded Loop Center Element
I recently read of one claim for greatly improved performance of a shieled loop when changing from RG59 to
3/4" hardline for the loop....same diameter. The conclusion was that the much larger center conductor of
the hardline was the reason. Is there any other evidence that might support this?
When I did my in-depth work on loop antennas, I found that as the diameter of the element (NOT the loop diameter) was increased, the efficiency improved. This was a result of the larger cross sectional area of the skin surface of the element, which results in lower bulk losses. The same result can be extrapolated to show that a larger diamer inner element of a shielded loop will have lower bulk losses, therefore better efficiency. Chris Trask N7ZWY / WDX3HLB Senior Member IEEE
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Shielded Loop Center Element
I recently read of one claim for greatly improved performance of a shieled loop when changing from RG59 to 3/4" hardline for the loop....same diameter. The conclusion was that the much larger center conductor of the hardline was the reason. Is there any other evidence that might support this?
Steve VE7SL
WEB: The VE7SL Radio Notebook:?
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Re: shielded loop question
Hi Chris. For sure it's a very good preamp but it implies too much coils for me. I have some?toroid but can't identify them. Unless you have one readily made for sale.?I forgot to mention that my band of interest is AM broadcast and LW. So, a preamp working from 100kHz to 2mHz is all I need. There is no longer AM station in my town. Overload is not an issue. Thanks Danny My blog: dannibou.wordpress.com
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Everett N4CY