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Old 5th March 2009, 07:54 PM   #11
Mark245 is offline Mark245  United States
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Ok here is a picture and schematic of what I am dealing with. Actually this is a class project for the lab I teach. We basically want to build a simple AM receiver for 1230kHz and when it works, turn it into a heterodyne receiver. We just wanted to make it as simple as possible at first.

All of the circuitry is on the breadboard. The antenna consists of magnet wire wrapped around a pizza box as shown in the photo. It has 7 loops. According to the Joe Carr formula, this will give an inductance of 61uH. So to resonate this antenna at 1230kHz, we need a 270pF cap. When I use these values, I get nothing from the speaker but like I say, when I touch the conductor from the antenna to the RF amp input, the radio station comes through pretty well. If I can get the radio to work without having to touch the antenna with my hand then it will be a success. Thanks for the advice so far and any other thoughts will be appreciated.

Thanks. Mark.
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Old 5th March 2009, 07:55 PM   #12
Mark245 is offline Mark245  United States
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The picture.
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Old 5th March 2009, 10:24 PM   #13
infinia is offline infinia  United States
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Hi
I think you should try to "balance the antenna" by using a buffer between the LC tank and the breadboard with a common mode choke.

Use smaller twin lead wire wrap about 15-20 turns around a ferrite toroid (26 AWG speaker zip cord is OK) and get a little more distance too.

If that fails checK the resonace and determine Q of the tank.
Sweep the generator with a small series resistor ( 100 ohm use for current). Find the peak Vpk and the 1/2 power points. 0.707Vpk.


edit>I'm sure the choke is going to fix it for you tho.

good luck
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Old 5th March 2009, 11:08 PM   #14
infinia is offline infinia  United States
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Analog engineers have to have a big old box-o-tricks to get by.
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Old 6th March 2009, 12:23 AM   #15
Mark245 is offline Mark245  United States
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Ok Thanks! I will try that tomorrow.
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Old 7th March 2009, 10:34 AM   #16
infinia is offline infinia  United States
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AM Long wire DX

Last year a guy in Quoddy Head, Maine (Eastern most point of land in the continental USA.) got recorded contacts in Saudi Arabia in the day.

Here is link to his Dx log http://www.kongsfjord.no/dl/DX-pedit...%20rev%208.pdf
scroll to last page for pictures and interesting story.


excerp
"It is a good thing I started early. The BOG's were operational by 3 pm EDT (1900 UTC) and I was doing a little casual listening to domestics in 6 kHz BW AM mode when I came across a big het on 1520. So I changed to 3 kHz BW USB and tuned 1521. There was Saudi Arabia virtually in the clear. In 20+ years of listening in Louisiana and Texas I gave gotten clear 1521 Saudi Arabia audio exactly once. To do it here at Quoddy Head, Maine at 3:15 pm EDT some 2.5 hours before sunset was amazing to me"
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Old 7th March 2009, 02:49 PM   #17
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Old 11th March 2009, 12:13 AM   #18
Mark245 is offline Mark245  United States
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Infinia, You mean put the RFC in parallel with the LC tank of the antenna right? I tried this with a 10000uH inductor. I still have to touch the antenna with my fingers at the input to the RF amp to make it work. However, it does make the signal sound a little better.

I also tried putting the RFC in series with the antenna. I assume this is not what you meant because it completely blocks the RF signal. Should I try using the wire wrapped around a torroid like you mentioned? Thanks for the help.
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Old 11th March 2009, 12:42 AM   #19
infinia is offline infinia  United States
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Hi
Yes use a common mode choke betwen the LC and the preamp

see link Fig 4 http://www.audiosystemsgroup.com/RFI-Ham.pdf
Like in Fig 4 but instead of coax you will use twin lead 26AWG speaker zip wire.
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Old 11th March 2009, 01:57 AM   #20
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Measurement of loop constants for a high Q LC tank circuit

These factors can be measured using a signal generator fed via a fairly high resistance (say 10 k) to the loop as shown in Figure 1. The voltage across the loop is monitored on a CRO (or perhaps a VTVM) via a high impedance probe. The signal generator frequency is adjusted for a peak in voltage at which the natural frequency is indicated.



Having measured the self-resonant frequency
As a further operation, Q factor can be measured using the same equipment, except that the resistance in series with the signal generator must be increased to around 500 kohm to prevent the Q being lowered by the signal source. With extra signal loss across the resistor, a high signal level and a sensitive CRO are needed. The procedure is simply to measure the frequencies either side of resonance which give 0.707 of the voltage at resonance. The Q factor is equal to the resonant frequency, divided by the difference between the two frequencies recorded.

Q=fc/BW where fc= freq maxima and, BW= f2 - f1

f1 =0,707 Vpk freq of low side -3dB point
f2 =0,707 Vpk freq of high side -3dB point

Figure 1 - Measurement of Loop Aerial constants
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