is it possible to connect a variable cap inside when loop is outside?

[st25r]

I have two pieces of about 20 feet of rg58 cable. I was wondering if I could take these two cables, hook the pickup coil to the antenna portion of my tuner and then have the main coil be tuned from inside with the 365pf variable cap. I tried it briefly the other day, but I could not get it to peak on a known station. Do I need to add capacitance?

 

[prcguy]

No. The main coil and the variable cap must connect together to form a parallel resonant circuit. You can't do that after the pickup coil.

 

[st25r]

Thank you prcguy for the quick answer. I'm still having some trouble understanding the 'why' behind it. So how I have it now is the main coil is connected to the capacitor that is mounted close to it, and the pickup coil is going to the tuner. Why wouldn't it work when I connected a longer length of coax to the coil and then connected to the variable cap inside? Is it trying to tune the coil and the coax? Wouldn't it still be in parallel with just longer wires attached? My apologies for these crazy question as I am trying to figure out why. Thanks!

 

[MUTNAV]

Thank you prcguy for the quick answer. I'm still having some trouble understanding the 'why' behind it. So how I have it now is the main coil is connected to the capacitor that is mounted close to it, and the pickup coil is going to the tuner. Why wouldn't it work when I connected a longer length of coax to the coil and then connected to the variable cap inside? Is it trying to tune the coil and the coax? Wouldn't it still be in parallel with just longer wires attached? My apologies for these crazy question as I am trying to figure out why. Thanks!

To me it sounds like you have it, you need to tune the resonant circuit, not the coax and the coil. I thought about this years ago (many years, just getting started).

There are ways of tuning loops remotely though.

Thanks
Joel

 

[prcguy]

If you could remote the variable capacitor from the coil with coax, not counting its on the wrong side of the pickup loop, you would have a huge amount of capacitance added to the circuit from the coax. For example RG-58 coax has about 25pf per foot, so 50ft would add about 1,250pf across the coil before you even connected your 365pf variable. That would be a problem because now you would have a variable capacitor of about 1,260pf to 1,625pf and way out of spec for your project, not to mention a bunch of other problems from doing this.

 

[st25r]

Oh wow that was an excellent explanation. It's certainly good to know about all of that extra capacitance which would explain about the stations not peaking when rotating the capacitor's shaft. Would it be possible to explain what you mean by the capacitor being on the wrong side of the pickup loop? How am I supposed to connect it? Thank you again, prcguy!

 

[st25r]

To me it sounds like you have it, you need to tune the resonant circuit, not the coax and the coil. I thought about this years ago (many years, just getting started).

There are ways of tuning loops remotely though.

Thanks
Joel

It would be cool to be able to tune it remotely. That was what I was trying to get at. An antenna tuner possibly?

 

[st25r]

If you could remote the variable capacitor from the coil with coax, not counting its on the wrong side of the pickup loop, you would have a huge amount of capacitance added to the circuit from the coax. For example RG-58 coax has about 25pf per foot, so 50ft would add about 1,250pf across the coil before you even connected your 365pf variable. That would be a problem because now you would have a variable capacitor of about 1,260pf to 1,625pf and way out of spec for your project, not to mention a bunch of other problems from doing this.

Hi prcguy, I forgot to quote you on the message above.
Would it be possible to explain what you mean by the capacitor being on the wrong side of the pickup loop? How am I supposed to connect it? Thank you again, prcguy!

 

[prcguy]

I'm more familiar with a ferrite loop antenna where there is a primary coil of wire around the ferrite in parallel with a tuning capacitor, usually 365pf variable. The inductance of the primary coil along with the capacitance determines the resonant frequency of the parallel LC circuit. Then there is a secondary coil, usually a few turns wound over the primary coil to couple a signal from the LC circuit to your radio. A variable capacitor across that coil will not change the resonant frequency much if at all.

You might have a larger loop with no ferrite and a variable capacitor to resonate it across a band of frequencies and that would act the same as the primary coil and capacitor in the above description. There should be a smaller pickup loop to couple signals off to the radio similar to the secondary loop on the ferrite antenna.

You can hook a small slow speed motor to your variable capacitor and run voltage up the coax to run the motor so you can remote the antenna. You would need a "bias tee" to insert the motor voltage into the coax and bring it back out near the antenna without disturbing the RF inside the coax.

Oh wow that was an excellent explanation. It's certainly good to know about all of that extra capacitance which would explain about the stations not peaking when rotating the capacitor's shaft. Would it be possible to explain what you mean by the capacitor being on the wrong side of the pickup loop? How am I supposed to connect it? Thank you again, prcguy!

 

[st25r]

I'm more familiar with a ferrite loop antenna where there is a primary coil of wire around the ferrite in parallel with a tuning capacitor, usually 365pf variable. The inductance of the primary coil along with the capacitance determines the resonant frequency of the parallel LC circuit. Then there is a secondary coil, usually a few turns wound over the primary coil to couple a signal from the LC circuit to your radio. A variable capacitor across that coil will not change the resonant frequency much if at all.

You might have a larger loop with no ferrite and a variable capacitor to resonate it across a band of frequencies and that would act the same as the primary coil and capacitor in the above description. There should be a smaller pickup loop to couple signals off to the radio similar to the secondary loop on the ferrite antenna.

You can hook a small slow speed motor to your variable capacitor and run voltage up the coax to run the motor so you can remote the antenna. You would need a "bias tee" to insert the motor voltage into the coax and bring it back out near the antenna without disturbing the RF inside the coax.

Thanks, prcguy! I have it on a ferrite rod. I apologize for not using the correct terminology. I do have the tuning capacitor in parallel on the primary coil and the secondary coil going to the tuner. It's a relief that it is hooked up correctly. I have on order one of those preamps you recommended to enhance the reception.

 

[MUTNAV]

This is another remotely tuned loop design, (I haven't made it but it seems reasonable and with a decent explanation).

I thought that there was a section on this site for just loop antennas (both commercial and home-made), but I haven't found it in a timely fashion.

Remotely-tuned loop antenna for LW and MW

Thanks
Joel

 

[st25r]

I'm more familiar with a ferrite loop antenna where there is a primary coil of wire around the ferrite in parallel with a tuning capacitor, usually 365pf variable. The inductance of the primary coil along with the capacitance determines the resonant frequency of the parallel LC circuit. Then there is a secondary coil, usually a few turns wound over the primary coil to couple a signal from the LC circuit to your radio. A variable capacitor across that coil will not change the resonant frequency much if at all.

Hi prcguy, the preamplifier board came in the mail today. I've hooked it up and there is hardly any noise coming out of the speakers. The tuning capacitor does not peak either. I attached the primary winding to the loop section on the board. Then attached the tuning capacitor to the RX2 and shield because I used a 9 volt battery for the power supply. The secondary coil I left alone. It goes straight to the tuner. Any thoughts on what I did wrong?

 

[prcguy]

The preamplifier would go between the secondary coil (pickup loop) and your receiver. You must leave the primary coil in parallel with the tuning capacitor. If the antenna was tuning and working before you attached the preamp then put it back like it was and connect the preamp output to your receiver and the preamp input to the secondary loop or pickup coil.

Hi prcguy, the preamplifier board came in the mail today. I've hooked it up and there is hardly any noise coming out of the speakers. The tuning capacitor does not peak either. I attached the primary winding to the loop section on the board. Then attached the tuning capacitor to the RX2 and shield because I used a 9 volt battery for the power supply. The secondary coil I left alone. It goes straight to the tuner. Any thoughts on what I did wrong?

 

[st25r]

The preamplifier would go between the secondary coil (pickup loop) and your receiver. You must leave the primary coil in parallel with the tuning capacitor. If the antenna was tuning and working before you attached the preamp then put it back like it was and connect the preamp output to your receiver and the preamp input to the secondary loop or pickup coil.

Thank you for the help with the connections. It definitely peaks. I think I will try it with MW and SW since where I am there are 2 sometimes 3 competing stations using the tuning capacitor.