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Using a CB Antenna with an SDR receiver #antennas #elmer #howto #elmer #howto #antennas


Joshua - KK6VHH
 

I have an SDR dongle and I just got an MCX to SO-259 adapter....

The dongle is receive only, I have a CB antenna, will that work?

Do I need to impedance match if I am just receiving?

Can I use Rev.C coax?

Maybe I an bring this up at tonight's net?

--
KK6VHH - Joshua

piratesinteepees.org
youtube.com/piratesinteepees


Jef - N5JEF
 

Joshua -

What kind of CB antenna did you get?  Probably a mobile?  Full quarter-wave whip, or loaded?

What do you want to receive?  I'm guessing not CB?

For receive-only, you can use almost any piece of wire to receive.  How long, how high, what kind of counterpoise will all be factors that depend on what bands you want to hear.

- Jef

On Thu, Oct 15, 2020 at 12:54 PM Joshua - KK6VHH <besneatte@...> wrote:
I have an SDR dongle and I just got an MCX to SO-259 adapter....

The dongle is receive only, I have a CB antenna, will that work?

Do I need to impedance match if I am just receiving?

Can I use Rev.C coax?

Maybe I an bring this up at tonight's net?

--
KK6VHH - Joshua

piratesinteepees.org
youtube.com/piratesinteepees


Joshua - KK6VHH
 

it's called a fire stick... about 3 foot long fiberglass pole with a considerable amount of wire wrapped around it and a spring base... like you see on 4x4s

being that it's SDR I can tune to and demodulate most any broadcasted signal... 

CB antenna is just sitting useless, so I am going to give it a try.

--
KK6VHH - Joshua

piratesinteepees.org
youtube.com/piratesinteepees


Jef - N5JEF
 

Joshua -

The three foot firestick will perform as a receiving antenna almost exactly the same as any three foot length of wire. A three foot length of wire without a ground plan will be most sensitive somewhere in the VHF region, depending on impedance matching.

Perhaps for further enlightenment:

"Tell me, Teacher, does the 3-ft Firestick have the Antenna-Nature?"

"The Antenna-Nature rests within all conductors, young padwan, ready to radiate or receive in proportion to electrical charge accelerated over lineal distance."

"But which is the True Path?  That of twists and turns of wire, or is it the Path through space of the superposition of fields of charge moving relativistically?"

"There are many Paths, padwan, and all are True within the appropriate context.  The aperture of the three foot Firestick is relative to its lineal length of three feet.  The electrical load as seen by the transmitter is as a 102 inch whip, but the coiling reduces its bandwidth.  Remember that there is No Free Lunch, and for any antenna design one may choose two, but not all three, of small size, efficiency, and bandwidth."

"But tell me, Teacher, what should I do?"

"Young padwan, keep in mind always your desired wavelength, then go forth and experiment!"

- Jef  N5JEF

On Sat, Oct 17, 2020 at 9:25 AM Joshua - KK6VHH <besneatte@...> wrote:
it's called a fire stick... about 3 foot long fiberglass pole with a considerable amount of wire wrapped around it and a spring base... like you see on 4x4s

being that it's SDR I can tune to and demodulate most any broadcasted signal... 

CB antenna is just sitting useless, so I am going to give it a try.

--
KK6VHH - Joshua

piratesinteepees.org
youtube.com/piratesinteepees


Aaron K6ABJ
 

“I understand there is no free lunch, but I was unaware that one may choose two of the three, desires, small size, efficiency, and bandwidth."

"Please tell me, Teacher, if I am willing to sacrifice bandwidth to obtain favorable efficiency and small size, what antenna(s) would be good for me?"

Aaron, K6ABJ

On Oct 17, 2020, at 10:44 AM, Jef - N5JEF <jef@...> wrote:

here is No Free Lunch, and for any antenna design one may choose two, but not all three, of small size, efficiency, and bandwidth."

"But tell me, Teacher, what should I do?"


Jef - N5JEF
 

The small transmitting loop, often described misleadingly as the "magnetic loop" antenna, with a circumference under ⅓ wavelength (in some applications, under ⅒ wavelength) is a practical example of an antenna that sacrifices bandwidth in favor of good efficiency and very small size.

With proper construction, a small transmitting loop for HF can nearly match (and in some environments, exceed) the efficiency and radiation pattern of the standard half-wave dipole, but with an instantaneous bandwidth of only a few to several kHz (tuneable to operate anywhere desired within the band.) 

Because such a short antenna is very high Q, it will have very high circulating current, therefore it is imperative for efficiency that resistive losses in construction be kept extremely low, but if this done, the efficiency can be very high and nearly all the RF is radiated rather than dissipated as heat.

For more information on small transmitting loops, I highly recommend this writeup:  

For more information on the fundamental tradeoff between small size, efficiency, and bandwidth:

- Jef  N5JEF











 


See 






On Sun, Oct 18, 2020 at 1:40 AM Aaron Jones <aaron@...> wrote:
“I understand there is no free lunch, but I was unaware that one may choose two  of the three, desires, small size, efficiency, and bandwidth."

"Please tell me, Teacher, if I am willing to sacrifice bandwidth to obtain favorable efficiency and small size, what antenna(s) would be good for me?"

Aaron, K6ABJ

> On Oct 17, 2020, at 10:44 AM, Jef - N5JEF <jef@...> wrote:
>
> here is No Free Lunch, and for any antenna design one may choose two, but not all three, of small size, efficiency, and bandwidth."
>
> "But tell me, Teacher, what should I do?"


Aaron K6ABJ
 

Much appreciated Jedi Master.

73

Aaron, K6ABJ

On Oct 18, 2020, at 7:07 AM, Jef Allbright <jef@...> wrote:


The small transmitting loop, often described misleadingly as the "magnetic loop" antenna, with a circumference under ⅓ wavelength (in some applications, under ⅒ wavelength) is a practical example of an antenna that sacrifices bandwidth in favor of good efficiency and very small size.

With proper construction, a small transmitting loop for HF can nearly match (and in some environments, exceed) the efficiency and radiation pattern of the standard half-wave dipole, but with an instantaneous bandwidth of only a few to several kHz (tuneable to operate anywhere desired within the band.) 

Because such a short antenna is very high Q, it will have very high circulating current, therefore it is imperative for efficiency that resistive losses in construction be kept extremely low, but if this done, the efficiency can be very high and nearly all the RF is radiated rather than dissipated as heat.

For more information on small transmitting loops, I highly recommend this writeup:  

For more information on the fundamental tradeoff between small size, efficiency, and bandwidth:

- Jef  N5JEF











 


See 






On Sun, Oct 18, 2020 at 1:40 AM Aaron Jones <aaron@...> wrote:
“I understand there is no free lunch, but I was unaware that one may choose two  of the three, desires, small size, efficiency, and bandwidth."

"Please tell me, Teacher, if I am willing to sacrifice bandwidth to obtain favorable efficiency and small size, what antenna(s) would be good for me?"

Aaron, K6ABJ

> On Oct 17, 2020, at 10:44 AM, Jef - N5JEF <jef@...> wrote:
>
> here is No Free Lunch, and for any antenna design one may choose two, but not all three, of small size, efficiency, and bandwidth."
>
> "But tell me, Teacher, what should I do?"


Skip - K6DGW
 

Jef points out a couple of advantages [and disadvantages] of small transmitting loops. The circulating current will be high with any appreciable power. The radiation resistance is in the milliohm ranges, and any resistance in the loop will dramatically lower the efficiency. It is tempting to build a loop as an octagon with copper pipe since the fittings are readily available, however the added resistance in the soldered joints can be as much or more than the radiation resistance, cutting the efficiency in half or more. Silver solder will ameliorate this a little, but a continuous length of copper pipe is much better. The voltages across the tuning capacitor will rise dramatically with higher power, well into the kilovolt range, requiring vacuum capacitors. Remembering that the RF current travels on the surface of the loop and is excluded from the inner region, the larger the pipe the better.

There are some decided advantages too. The primary coupling mode for small loops is via the H-field and as a result, they tend to be insensitive to "ground" and surrounding objects so long as those objects have a permeability close to 1.0 ... I sit under my Alexloop which is on a tripod, and the tuning capacitor is just over my head. There is little if any advantage to putting a small loop on a tower. The loop is broadly bi-directional in the plane of the loop and the two nulls [orthogonal to the loop plane] are quite sharp.

Most loops use magnetic coupling, feeding the power through a smaller loop inside the main loop. It is a resonant transformer, and the key word there is "resonant." The loop must be operated exactly at resonance for power transfer to take place. A 1:4 or 1:5 ratio between the diameter of the feed and main loops will provide close to a 50 ohm load to the transmitter. The process is, 1) Bypass the ATU; 2) At low power, tune the loop for zero reactance; 3) Engage the ATU and let it find a match. "Getting the loop close and letting the ATU match the rest" is a recipe for a very effective dummy load. A phase detector is a real advantage, just tune the loop for zero phase difference between the voltage and current. In fact, that's what is usually used for automatic, motor-driven vacuum capacitors on transmitting loops.

https://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx is one of many sites with small magnetic loop calculators. Try a few designs out and you'll see that, at 100 W, you need to keep your distance! <K6DGW>


Aaron K6ABJ
 

Thank you Skip for the link to the loop antenna radiation calculator.  I was looking for that, but couldn't find it! Are you operating your Alex Loop in a horizontal position so the null is towards you? Unless a person is operating from a second story or higher, as I understand its best to have a loop antenna in a vertical orientation. I am trying to picture your setup, and how I will do mine when I get a Magnetic Loop Antenna.

Jeff piqued my interest in Magnetic Loop Antennas, and the more I have learned the more interested I have become. Being a high Q antenna, they have a VERY narrow bandwidth, but their OUTSTANDING efficiency, small size, and lack of need for a mast make these antennas VERY appealing to me. I have enjoyed learning about and researching Loop Antennas the last few days! These antennas wouldn’t work well in a contest for Searching and Pouncing, but if Running (which I prefer to do) it would do great!

Their extremely high voltage, current and non-ionizing radiation creates real safety concerns. These issues however are a good sign that the antenna will get me the DX I want! I will simply mitigate the safety issues by securely supporting the antenna and ensuring I and others maintain enough distance from it while transmitting. The challenge before me now is how do I get one!?

Building one is possibly the most affordable option, but sourcing a capable capacitor is a significant challenge! Building the loop using 3/8 inch flexible copper pipe would be a good material as it is relatively thick and could be made out of a single piece. Perhaps cut the ends using a dremel to create flat copper tabs to directly attach the loop to the capacitor.

MFJ’s products have significant workmanship/assembly issues, so I generally avoid MFJ, but they have three High-Efficiency Loop Tuners to choose from that would solve the capacitor sourcing problem. The only difference between the MFJ 933 ($209.95) and 935B ($249.59) is that the 935B includes an Antenna Current Meter. The MFJ 936B ($299.85) is like the others, but it takes up more space, includes an SWR Meter, a Current Meter, and adds 60 Meters. For me, I think the 935B is the best option. The nice thing about going with one of these as opposed to something else is that they are good for up to 150 watts, 10-40 meters, and I can make my own loop. By making my own loop I can experiment with various materials, can have a greater variety of loop circumferences allowing for greater performance and more bands, and I can roll the loop up for greater portability. I do however see two downsides to going this route: 1) The MFJ 1788 Super Hi-Q Loop Antenna ($559.95) comes with a loop that is an inch thick so would significantly improve performance over a loop I could make myself, and 2) The MFJ 1788 comes with a controller that allows the operator to tune the Antenna from afar, something that seems like a very good idea given the extreme voltage and non-ionizing radiation that occurs with these antennas.

The MFJ 1788 is rated for 150 watts, just like the loop tuners, but it is only good for 15-40 meters. Whether the efficiency of having a thicker loop would more than make up for the ability to have loops for each band, I do not know. Aside from the added cost, the other potential problem with the 1788 is that it relies on an electronic tuner and motor, which given that its made by MFJ could fail. Still, the ability to be able to tune the antenna from afar given the amount of radiation that is involved, if cost wasn’t a factor I think I would go for the 1788 over the 935B.

Other loop antennas that were considered: The AlexLoop ($399.95) appears to be very well constructed, comes with a nice bag, and is 10-40 meters. The downside is that it is only rated for 20 watts. Still, given the amount of power Magnetic Loops put out 20 watts isn’t bad, especially if you are using it for portable. The price is very competitive, but given that I want to use my loop antenna for serious contesting in addition to portable operations, the MFJ 935B for me remains the leader.

The Alpha Antenna ($500.00), is another option. It too appears to be a well-made antenna and comes with a bag. It also comes with a small tripod. It is rated at 100 watts for 10-40 meters, and 20 Watts 40-80 meters. It is twice as much money as the 935B, but the advantage to it or the AlexLoop over the 935B is ease of use. The capacitors are smaller and lighter than that of the 935B, making them able to be supported by a small tri-pod. This however raises a very important question, given its larger and heavier size does the capacitor of the 935B perform better? I think the answer to this question determines which wins out, the MFJ 935B or the Alpha.

I think there is only one more antenna to consider, and that is the Ciro Mazzoni Automatic Magnetic Loop Antenna. It is one incredible Antenna, weighing an astonishing 44lbs is isn’t good for SOTA, but it is VERY well made, can be remotely tuned, is rated for 800 watts on 20-30 meters, and 300 watts on 40-80 meters. The rub is that it costs $2,479.99. That is a lot of bread, unless you consider the cost of the land required to erect a comparable 80 meter dipole!    

On Mon, Oct 19, 2020 at 12:17 PM Skip - K6DGW <k6dgw@...> wrote:
Jef points out a couple of advantages [and disadvantages] of small transmitting loops.  The circulating current will be high with any appreciable power.  The radiation resistance is in the milliohm ranges, and any resistance in the loop will dramatically lower the efficiency.  It is tempting to build a loop as an octagon with copper pipe since the fittings are readily available, however the added resistance in the soldered joints can be as much or more than the radiation resistance, cutting the efficiency in half or more.  Silver solder will ameliorate this a little, but a continuous length of copper pipe is much better.  The voltages across the tuning capacitor will rise dramatically with higher power, well into the kilovolt range, requiring vacuum capacitors.  Remembering that the RF current travels on the surface of the loop and is excluded from the inner region, the larger the pipe the better.

There are some decided advantages too.  The primary coupling mode for small loops is via the H-field and as a result, they tend to be insensitive to "ground" and surrounding objects so long as those objects have a permeability close to 1.0 ... I sit under my Alexloop which is on a tripod, and the tuning capacitor is just over my head.  There is little if any advantage to putting a small loop on a tower.  The loop is broadly bi-directional in the plane of the loop and the two nulls [orthogonal to the loop plane] are quite sharp.

Most loops use magnetic coupling, feeding the power through a smaller loop inside the main loop.  It is a resonant transformer, and the key word there is "resonant."  The loop must be operated exactly at resonance for power transfer to take place.  A 1:4 or 1:5 ratio between the diameter of the feed and main loops will provide close to a 50 ohm load to the transmitter.  The process is, 1) Bypass the ATU; 2) At low power, tune the loop for zero reactance; 3) Engage the ATU and let it find a match.  "Getting the loop close and letting the ATU match the rest" is a recipe for a very effective dummy load.  A phase detector is a real advantage, just tune the loop for zero phase difference between the voltage and current.  In fact, that's what is usually used for automatic, motor-driven vacuum capacitors on transmitting loops.

https://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx is one of many sites with small magnetic loop calculators.  Try a few designs out and you'll see that, at 100 W, you need to keep your distance! <K6DGW>






Jef - N5JEF
 

Here's a picture of my 10-ft diameter loop for 40-60-80 meters, a work in progress.

image.png

image.png



On Tue, Oct 20, 2020 at 1:18 AM Aaron K6ABJ <aaron@...> wrote:

Thank you Skip for the link to the loop antenna radiation calculator.  I was looking for that, but couldn't find it! Are you operating your Alex Loop in a horizontal position so the null is towards you? Unless a person is operating from a second story or higher, as I understand its best to have a loop antenna in a vertical orientation. I am trying to picture your setup, and how I will do mine when I get a Magnetic Loop Antenna.

Jeff piqued my interest in Magnetic Loop Antennas, and the more I have learned the more interested I have become. Being a high Q antenna, they have a VERY narrow bandwidth, but their OUTSTANDING efficiency, small size, and lack of need for a mast make these antennas VERY appealing to me. I have enjoyed learning about and researching Loop Antennas the last few days! These antennas wouldn’t work well in a contest for Searching and Pouncing, but if Running (which I prefer to do) it would do great!

Their extremely high voltage, current and non-ionizing radiation creates real safety concerns. These issues however are a good sign that the antenna will get me the DX I want! I will simply mitigate the safety issues by securely supporting the antenna and ensuring I and others maintain enough distance from it while transmitting. The challenge before me now is how do I get one!?

Building one is possibly the most affordable option, but sourcing a capable capacitor is a significant challenge! Building the loop using 3/8 inch flexible copper pipe would be a good material as it is relatively thick and could be made out of a single piece. Perhaps cut the ends using a dremel to create flat copper tabs to directly attach the loop to the capacitor.

MFJ’s products have significant workmanship/assembly issues, so I generally avoid MFJ, but they have three High-Efficiency Loop Tuners to choose from that would solve the capacitor sourcing problem. The only difference between the MFJ 933 ($209.95) and 935B ($249.59) is that the 935B includes an Antenna Current Meter. The MFJ 936B ($299.85) is like the others, but it takes up more space, includes an SWR Meter, a Current Meter, and adds 60 Meters. For me, I think the 935B is the best option. The nice thing about going with one of these as opposed to something else is that they are good for up to 150 watts, 10-40 meters, and I can make my own loop. By making my own loop I can experiment with various materials, can have a greater variety of loop circumferences allowing for greater performance and more bands, and I can roll the loop up for greater portability. I do however see two downsides to going this route: 1) The MFJ 1788 Super Hi-Q Loop Antenna ($559.95) comes with a loop that is an inch thick so would significantly improve performance over a loop I could make myself, and 2) The MFJ 1788 comes with a controller that allows the operator to tune the Antenna from afar, something that seems like a very good idea given the extreme voltage and non-ionizing radiation that occurs with these antennas.

The MFJ 1788 is rated for 150 watts, just like the loop tuners, but it is only good for 15-40 meters. Whether the efficiency of having a thicker loop would more than make up for the ability to have loops for each band, I do not know. Aside from the added cost, the other potential problem with the 1788 is that it relies on an electronic tuner and motor, which given that its made by MFJ could fail. Still, the ability to be able to tune the antenna from afar given the amount of radiation that is involved, if cost wasn’t a factor I think I would go for the 1788 over the 935B.

Other loop antennas that were considered: The AlexLoop ($399.95) appears to be very well constructed, comes with a nice bag, and is 10-40 meters. The downside is that it is only rated for 20 watts. Still, given the amount of power Magnetic Loops put out 20 watts isn’t bad, especially if you are using it for portable. The price is very competitive, but given that I want to use my loop antenna for serious contesting in addition to portable operations, the MFJ 935B for me remains the leader.

The Alpha Antenna ($500.00), is another option. It too appears to be a well-made antenna and comes with a bag. It also comes with a small tripod. It is rated at 100 watts for 10-40 meters, and 20 Watts 40-80 meters. It is twice as much money as the 935B, but the advantage to it or the AlexLoop over the 935B is ease of use. The capacitors are smaller and lighter than that of the 935B, making them able to be supported by a small tri-pod. This however raises a very important question, given its larger and heavier size does the capacitor of the 935B perform better? I think the answer to this question determines which wins out, the MFJ 935B or the Alpha.

I think there is only one more antenna to consider, and that is the Ciro Mazzoni Automatic Magnetic Loop Antenna. It is one incredible Antenna, weighing an astonishing 44lbs is isn’t good for SOTA, but it is VERY well made, can be remotely tuned, is rated for 800 watts on 20-30 meters, and 300 watts on 40-80 meters. The rub is that it costs $2,479.99. That is a lot of bread, unless you consider the cost of the land required to erect a comparable 80 meter dipole!    

On Mon, Oct 19, 2020 at 12:17 PM Skip - K6DGW <k6dgw@...> wrote:
Jef points out a couple of advantages [and disadvantages] of small transmitting loops.  The circulating current will be high with any appreciable power.  The radiation resistance is in the milliohm ranges, and any resistance in the loop will dramatically lower the efficiency.  It is tempting to build a loop as an octagon with copper pipe since the fittings are readily available, however the added resistance in the soldered joints can be as much or more than the radiation resistance, cutting the efficiency in half or more.  Silver solder will ameliorate this a little, but a continuous length of copper pipe is much better.  The voltages across the tuning capacitor will rise dramatically with higher power, well into the kilovolt range, requiring vacuum capacitors.  Remembering that the RF current travels on the surface of the loop and is excluded from the inner region, the larger the pipe the better.

There are some decided advantages too.  The primary coupling mode for small loops is via the H-field and as a result, they tend to be insensitive to "ground" and surrounding objects so long as those objects have a permeability close to 1.0 ... I sit under my Alexloop which is on a tripod, and the tuning capacitor is just over my head.  There is little if any advantage to putting a small loop on a tower.  The loop is broadly bi-directional in the plane of the loop and the two nulls [orthogonal to the loop plane] are quite sharp.

Most loops use magnetic coupling, feeding the power through a smaller loop inside the main loop.  It is a resonant transformer, and the key word there is "resonant."  The loop must be operated exactly at resonance for power transfer to take place.  A 1:4 or 1:5 ratio between the diameter of the feed and main loops will provide close to a 50 ohm load to the transmitter.  The process is, 1) Bypass the ATU; 2) At low power, tune the loop for zero reactance; 3) Engage the ATU and let it find a match.  "Getting the loop close and letting the ATU match the rest" is a recipe for a very effective dummy load.  A phase detector is a real advantage, just tune the loop for zero phase difference between the voltage and current.  In fact, that's what is usually used for automatic, motor-driven vacuum capacitors on transmitting loops.

https://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx is one of many sites with small magnetic loop calculators.  Try a few designs out and you'll see that, at 100 W, you need to keep your distance! <K6DGW>






Skip - K6DGW
 

A small loop mounted horizontally behaves like an E-field antenna and needs to be elevated. I tried it once out a 4th floor hotel window. Didn't work well.

All the loop needs is a capacitor to tune it to resonance. Anything else just increases the non-radiating losses. Based on what the calculator finds for the voltage[s], it can be a any transmitting cap with sufficient voltage rating. Most use vacuum variables since it is outside at the loop and even if in a box, is still exposed to outdoor humidity. One approach I've used is to fit a smaller copper pipe into a piece of Schedule 40 or 80 PVC and then fit that coaxially into a larger copper pipe. A long threaded rod then moves the inner pipe in and out. I used wide copper strap for the connections for more surface area. The advantage is that you can get away without needing a reduction gear box, I had an 18 RPM reversible DC motor that just drove the threaded rod until it was at zero phase difference. Do not use ABS pipe for the dielectric, it isn't a dielectric. You can test plastic parts by putting them in a microwave and nuking them for varying times. Any that get warm are not suitable.

Regarding conductor diameter and thickness: The RF current is confined to the outer surface ... thickness doesn't add any benefits except mechanically. The larger the surface area of the loop, the lower the loss. Most of the loss in a well-built loop will be in the connections to the capacitor. Regardless of how you do it however, the loop MUST operate at resonance if you intend to work anyone with it. I can sit under my Alexloop [making it tunable without having to move back and forth] because I run my K2 at 5-7 watts when in the field. If you're running 100+ watts, you should tune it remotely for safety reasons. 73,

Fred ["Skip"] K6DGW
Sparks, NV


Jef - N5JEF
 



On Tue, Oct 20, 2020 at 6:05 AM Jef - N5JEF via groups.io <jef=jefallbright.net@groups.io> wrote:
Here's a picture of my 10-ft diameter loop for 40-60-80 meters, a work in progress.

image.png

image.png

Here's the flanges for attachment to the vacuum variable capacitor:

image.png
 


Doug - W2VX
 

I have built several "Mag Loop" antennas.

here is the best online calculator I have found for design.
https://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx

The best results came from using 16ft of 6" aluminum flashing, a 3-50pf vac capacitor rated at 25kv.
The 6" flashing produces a equivalent conductor diameter around 3.5" for efficiency of about -0.2dB.
I prepare the aluminum connection points by smearing antiox paste on the surface and then 100grit sand paper to remove the AlOxide.  Contact me for more details on this if interested. 

I get about 25-30kHz bandwidth across the 20meter band and can tune from 14-14.3mHz by changing the capacitor in the range of ~ 20-23pf.  Remote tuning accomplished with a 1rpm geared motor and a DPDT toggle switch for polarity control.  Simple really.  The 20meter version is 5ft diameter and works a good if not better than my 20meter dipole invee.  Contacts worldwide.  Lower noise.  Sits on top of two plastic saw horses for ground clearance.  It has nulls perpendicular to the loop.  Rotation in real time is IMPOSSIBLE. 

I've build a few of these in an attempt to tune to multiple bands but found this to be problematic. Self resonance and low efficiency. 

Last year I attempted a  40-80 meter version.  Crazy big.

I've had my fun and learned alot. I would be willing to sell the 20meter version for recovery cost of the capacitor and motor.  $160.   Happy to demo if that is needed.

Bring a truck to get it. Light weight but Big.

W2VX - 530-368-5706


On 10/20/20 6:53 PM, Jef - N5JEF wrote:


On Tue, Oct 20, 2020 at 6:05 AM Jef - N5JEF via groups.io <jef=jefallbright.net@groups.io> wrote:
Here's a picture of my 10-ft diameter loop for 40-60-80 meters, a work in progress.

image.png

image.png

Here's the flanges for attachment to the vacuum variable capacitor:

image.png
 
-- 
Call sign W2VX
Digital PSK31 FT8 FT8call 20Mtr 40Mtr
D-Star XRF002a APRS northwest.aprs2.net
DMR monitoring TG310
70cm 2,6,10,20,40Mtr
Monitor W6EK Repeater - 145.430 -600, PL162.2 
Rigs IC7100,IC7300
Location : Near Sacramento CA

--
Doug W2VX
w2vx@...
message 530 368 5703
Auburn CA


Jef - N5JEF
 

Doug -

Nice work, and makes sense too!

Thanks for sharing this.

- Jef  N5JEF



On Tue, Oct 27, 2020 at 1:06 PM Doug - W2VX <w2vx@...> wrote:

I have built several "Mag Loop" antennas.

here is the best online calculator I have found for design.
https://www.66pacific.com/calculators/small-transmitting-loop-antenna-calculator.aspx

The best results came from using 16ft of 6" aluminum flashing, a 3-50pf vac capacitor rated at 25kv.
The 6" flashing produces a equivalent conductor diameter around 3.5" for efficiency of about -0.2dB.
I prepare the aluminum connection points by smearing antiox paste on the surface and then 100grit sand paper to remove the AlOxide.  Contact me for more details on this if interested. 

I get about 25-30kHz bandwidth across the 20meter band and can tune from 14-14.3mHz by changing the capacitor in the range of ~ 20-23pf.  Remote tuning accomplished with a 1rpm geared motor and a DPDT toggle switch for polarity control.  Simple really.  The 20meter version is 5ft diameter and works a good if not better than my 20meter dipole invee.  Contacts worldwide.  Lower noise.  Sits on top of two plastic saw horses for ground clearance.  It has nulls perpendicular to the loop.  Rotation in real time is IMPOSSIBLE. 

I've build a few of these in an attempt to tune to multiple bands but found this to be problematic. Self resonance and low efficiency. 

Last year I attempted a  40-80 meter version.  Crazy big.

I've had my fun and learned alot. I would be willing to sell the 20meter version for recovery cost of the capacitor and motor.  $160.   Happy to demo if that is needed.

Bring a truck to get it. Light weight but Big.

W2VX - 530-368-5706


On 10/20/20 6:53 PM, Jef - N5JEF wrote:


On Tue, Oct 20, 2020 at 6:05 AM Jef - N5JEF via groups.io <jef=jefallbright.net@groups.io> wrote:
Here's a picture of my 10-ft diameter loop for 40-60-80 meters, a work in progress.

image.png

image.png

Here's the flanges for attachment to the vacuum variable capacitor:

image.png
 
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Call sign W2VX
Digital PSK31 FT8 FT8call 20Mtr 40Mtr
D-Star XRF002a APRS northwest.aprs2.net
DMR monitoring TG310
70cm 2,6,10,20,40Mtr
Monitor W6EK Repeater - 145.430 -600, PL162.2 
Rigs IC7100,IC7300
Location : Near Sacramento CA

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Doug W2VX
w2vx@...
message 530 368 5703
Auburn CA