I am building a big passive AM loop antenna. 42x29", 17 turns of 24 ga CCA twisted wire, 1/4" spacing, feedpoint at the bottom so it is horizontally polarized. it measured 637 uH so I used the 14-125 pF variable capacitor in series. inductively coupled to the radio with a loose wire in series with the antenna, 7 turns wrapped around the radio.

it's not receiving stations very well and the VR has little to no effect. the antenna is about useless.

however, when I tune the radio to 1400 KHz and adjust the VR, it brings in 550 KHz. (very strong local station) double checked everything (antenna only, induction wire detached) and got L= 636 uH, C= 106 pF.

with those the resonance frequency should be 612 KHz, but maybe my LCR meter is a little off, there is capacitance between the turns, the inductive coupling loop adds L, etc. it's in the neighborhood of 550.

then I tuned the radio to 1395 and adjusting VR wouldn't bring in anything.

so why does the radio receive 550 when tuned to 1400?

 

What type/model of receiver is it? Can you post a clear picture of how it's connected around the radio.


It could be a superheterodyne image but it's impossible to say exactly with no information.

 

In some old radios, a variable capacitor used a reduction pulley ratio 4:1 that reversed the direction however the fine tuning was much improved.
This string pulley reduction allowed a higher level of precise control. Many radios in those days were home made. There have been configurations that fixed the problems, as time went on most radios were mass produced and that was no longer a problem.
Intuitively most enthusiast turned the knob in the wrong direction during the period of homemade radios, the variable capacitor company installed the alternating fixed plates facing the opposite direction for those DIY hay days that artifact regarding that orientation has remained as a standard.

additionally edit
A magnetic loop uses a variable capacitor to peak the loop antenna and can be measured with and without the receiver on,

 

The IF (intermediate frequency) is commonly 455kHz and sometimes 465kHz. I have never encountered IF at 425kHz.
If your superheterodyne radio has IF of 425kHz that would be unusual. Also there is something else that does not add up.

Here is how a superhet radio works.

Suppose your local oscillator is set to 975kHz.

When mixed with a received RF at 550kHz, there will be two signals, one at (975-550)kHz = 425kHz
and another at (975+550)kHz = 1525kHz

When mixed with a received RF at 1400kHz, there will be two signals, one at (1400-975)kHz = 425kHz
and another at (1400+975)kHz = 2375kHz

If the IF amplifier is set to receive 425kHz, then the receiver will capture both RF signals at 550kHz and 1400kHz.

Secondly, all the AM radios I have worked on puts the local oscillator at RF + IF, i.e. above the MW band.
You must have a special radio.

 

The IF (intermediate frequency) is commonly 455kHz and sometimes 465kHz. I have never encountered IF at 425kHz.
If your superheterodyne radio has IF of 425kHz that would be unusual. Also there is something else that does not add up.

Here is how a superhet radio works.

Suppose your local oscillator is set to 975kHz.

When mixed with a received RF at 550kHz, there will be two signals, one at (975-550)kHz = 425kHz
and another at (975+550)kHz = 1525kHz

When mixed with a received RF at 1400kHz, there will be two signals, one at (1400-975)kHz = 425kHz
and another at (1400+975)kHz = 2375kHz

If the IF amplifier is set to receive 425kHz, then the receiver will capture both RF signals at 550kHz and 1400kHz.

Secondly, all the AM radios I have worked on puts the local oscillator at RF + IF, i.e. above the MW band.
You must have a special radio.

This is the same guy that was talking about making a SW receiver from the AM radio so who knows how much the RF, LO and IF have been retuned.

 

The simplest is add some sort of VNA to antenna, for example Nano-VNA capable to scan up from the 50 KHz up to 6 GHz. Antenna may have short-circuit inside, capacitor may have a short circuit between plates, the capacitor may happen be designed for woeful low frequency or damn lossy etc etc

 

The IF (intermediate frequency) is commonly 455kHz and sometimes 465kHz. I have never encountered IF at 425kHz.
If your superheterodyne radio has IF of 425kHz that would be unusual. Also there is something else that does not add up.

Here is how a superhet radio works.

Suppose your local oscillator is set to 975kHz.

When mixed with a received RF at 550kHz, there will be two signals, one at (975-550)kHz = 425kHz
and another at (975+550)kHz = 1525kHz

When mixed with a received RF at 1400kHz, there will be two signals, one at (1400-975)kHz = 425kHz
and another at (1400+975)kHz = 2375kHz

If the IF amplifier is set to receive 425kHz, then the receiver will capture both RF signals at 550kHz and 1400kHz.

Secondly, all the AM radios I have worked on puts the local oscillator at RF + IF, i.e. above the MW band.
You must have a special radio.

I checked the radio's LO frequency with an RF signal generator. set the freq to 455 kHz and waved the antenna around the case so the LO picks it up. nothing. did a sweep +/- 10 kHz, and it turns out the LO picks up 422.5 kHz. did the same thing with 2x frequency, the LO picks up 845 kHz. verified the radio or RF signal generator isn't out of whack by setting the RF to various freqs and picking it up on the radio at the same freq. both are consistent.


it's one of these.

 

The simplest is add some sort of VNA to antenna, for example Nano-VNA capable to scan up from the 50 KHz up to 6 GHz. Antenna may have short-circuit inside, capacitor may have a short circuit between plates, the capacitor may happen be designed for woeful low frequency or damn lossy etc etc

no signal from the antenna was resolved by checking the connections. a jumper to the VC was loose. fixed that, now it gets up to +30 dB gain on some freqs. as for receiving 550 at 1400 I have not been able to duplicate that. don't know why it did it before but it doesn't do it now. whatever.

 

no signal from the antenna was resolved by checking the connections. a jumper to the VC was loose. fixed that, now it gets up to +30 dB gain on some freqs. as for receiving 550 at 1400 I have not been able to duplicate that. don't know why it did it before but it doesn't do it now. whatever.

With the LO generating 975kHz, the IF amplifier would capture RF at both 550kHz and 1400kHz. It is the job of the input stage to attenuate one or the other, i.e. to select the desired frequency. If your tuning capacitor was not functional, that would explain what you experienced.

 

RE:""a jumper to the VC was loose. fixed that, now it gets up to""
This moment on my table stays antenna switch relay from mate`s sailboat (new cost about $1200). The problem was just due the bad contact in 25W 160 MHz) cable, now the switching pin power-diode is burned and if it have component name in clear Chinese language and no way to get the datasheet to find the right substitute, thus I have a large problem what to solder instead of it.