# Looking for resistor value and capacitor value for passive low pass filter remove RF

#### dietermoreno

Dec 30, 2012
238
While I am at it building a VHF set and VHF dipole antenna, I think I want it to receive aviation band as well.

Since I am using alligator clips instead of sodering, switching bands from FM to aviation is as simple as adding an extra tap on the inductor so that only 3 coils are connected instead of 4 coils and disconnect the slope detection wire lead in addition to select less plates on the variable capacitor.

I don't have any commercially produced radios that can receive aviation band, so I build my own!

I got the idea of an aviation band crystal set that can switch back and forth between FM and aviation band from this schematic: http://www.pcs-electronics.com/schematics/aircraft.pdf

You can't use cell phones or any form of radios while on commercial airliners, but I wonder if crystal sets would be allowed. A 37,000 foot tall antenna would certainly allow you to receive many stations on a crystal set lol.

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#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
While I am at it building a VHF set and VHF dipole antenna, I think I want it to receive aviation band as well.

Since I am using alligator clips instead of sodering, switching bands from FM to aviation is as simple as adding an extra tap on the inductor so that only 3 coils are connected instead of 4 coils and disconnect the slope detection wire lead in addition to select less plates on the variable capacitor.

Someone has to enlighten you so it might as well be me. Circuit design at VHF frequencies takes on new dimensions. Alligator jumpers clipped to coil taps in the VHF bands isn't going to cut it. A 1/4" length of wire can spell the difference between a tuned circuit and an detuned one. Forget Alligator jumpers in the VHF band, In fact forget about VHF altogether.

Chris

#### dietermoreno

Dec 30, 2012
238
Oh okay, so in the MW band no soldering is needed and it is acceptable to use tapped coils and alligator jumper clips, but in the VHF band just the length of the tap and the length of the alligator jumper clip is enough to cause the circuit to resonate at a lower frequency than it is supposed to.

So in the VHF band, every connection of the circuit needs to be as short as possible and soldering is necessary.

So crystal sets in the VHF band need to be more permanent rather than the continously recyled parts MW crystal set.

So in sets that are not super heterodyne, above SW there is no such thing as switching bands.

So an advantage of the super heterodyne receiver is that every band is stepped down to the same frequency (IF) to allow you to switch bands using the same inductor and the same variable capacitor.

Oh okay, so in the MW band no soldering is needed and it is acceptable to use tapped coils and alligator jumper clips, but in the VHF band just the length of the tap and the length of the alligator jumper clip is enough to cause the circuit to resonate at a lower frequency than it is supposed to.

So in the VHF band, every connection of the circuit needs to be as short as possible and soldering is necessary.

So crystal sets in the VHF band need to be more permanent rather than the continously recyled parts MW crystal set.

So in sets that are not super heterodyne, above SW there is no such thing as switching bands.

So an advantage of the super heterodyne receiver is that every band is stepped down to the same frequency (IF) to allow you to switch bands using the same inductor and the same variable capacitor.

And here some ones with both AM and FM:

That's weird, I see in the last image that the AM and the FM have two completely different circuits, and there are different antenna and different inductors. The AM circuit doesn't even have an antenna really and just uses the MW inductor inside a Ferrite rod as the antenna and the MW inductor has hundreds of coils. The FM circuit has a telescopic antenna and has a VHF inductor with 7 coils.

So I guess I'm wrong, I guess the super heterodyne receiver doesn't let you use the same inductor for all bands. I told you so when I told you that my grandma's clock radio was working for AM but not FM and I opened it up and saw that the inductor with only a few coils was loose and alligator clips worked to make the connection and made FM work. Alligator clips worked for that clock radio when onlry 2 alligator clips were used, but alligator clips should not be used to make every connection in the circuit, in fact alligator clips shouldn't be used to make any connection in the circuit.

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#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Oh okay, so in the MW band no soldering is needed and it is acceptable to use tapped coils and alligator jumper clips, but in the VHF band just the length of the tap and the length of the alligator jumper clip is enough to cause the circuit to resonate at a lower frequency than it is supposed to.
So in the VHF band, every connection of the circuit needs to be as short as possible and soldering is necessary.
First part absolutely true. Second part..generally true.
So crystal sets in the VHF band need to be more permanent rather than the continously recyled parts MW crystal set.
So in sets that are not super heterodyne, above SW there is no such thing as switching bands.
So an advantage of the super heterodyne receiver is that every band is stepped down to the same frequency (IF) to allow you to switch bands using the same inductor and the same variable capacitor.
(A) The key advantages of a Super-Het are greater selectivity in the IF strip, higher gain per stage in the IF strip and no need for user tunable components in the IF strip.

Oh okay, so in the MW band no soldering is needed and it is acceptable to use tapped coils and alligator jumper clips, but in the VHF band just the length of the tap and the length of the alligator jumper clip is enough to cause the circuit to resonate at a lower frequency than it is supposed to.
Generally true.
So in the VHF band, every connection of the circuit needs to be as short as possible and soldering is necessary.
(B) True but there can be solderless connections. These include VHF rated connectors. Special care in design must be employed for any switching that's in the signal and osc paths.
So crystal sets in the VHF band need to be more permanent rather than the continously recyled parts MW crystal set.
So in sets that are not super heterodyne, above SW there is no such thing as switching bands.
So an advantage of the super heterodyne receiver is that every band is stepped down to the same frequency (IF) to allow you to switch bands using the same inductor and the same variable capacitor.

That's weird, I see in the last image that the AM and the FM have two completely different circuits, and there are different antenna and different inductors. The AM circuit doesn't even have an antenna really and just uses the MW inductor inside a Ferrite rod as the antenna and the MW inductor has hundreds of coils. The FM circuit has a telescopic antenna and has a VHF inductor with 7 coils.

As I've stated previously spend some time in our Tutorial section. Also read as much radio theory as you can absorb.
So I guess I'm wrong, I guess the super heterodyne receiver doesn't let you use the same inductor for all bands. I told you so when I told you that my grandma's clock radio was working for AM but not FM and I opened it up and saw that the inductor with only a few coils was loose and alligator clips worked to make the connection and made FM work. Alligator clips worked for that clock radio when onlry 2 alligator clips were used, but alligator clips should not be used to make every connection in the circuit, in fact alligator clips shouldn't be used to make any connection in the circuit.

AG jumpers probably worked because you were messing around with FM antenna input where you're not detuning anything.

Chris

#### dietermoreno

Dec 30, 2012
238
Okay, from the last image I posted, it appears that the super het has the inductors before the down conversion to the IF, and it appears that there is only one variable capacitor and actually both circuits have the RF gain stages on all the time just there is a switch to connect the demodulated RF to the audio amp which is what actually switches from FM to AM since audio frequencies will handle a simple switch with no problems of detuning, correct?

So if I was to use any alligator jumper cables, the only place they should be used is to connect to the audio output jack if I was to build both aviation and FM crystal set on the same wood board substrate with 2 slightly different circuits (I'm not going to do this because I want to start off with only having to find components for one circuit), correct?

So bottom line, with crystal sets could switch bands in VHF, just 2 different circuits are needed with alligator jumper cables that allow you to switch which circuit is connected to the audio output jack, correct?

Also could the antenna connection use the jumper cables and use 2 separate variable capacitors that are each exactly the same?

I think really the switch should be at the antenna if the same antenna is used.

To make what I'm saying clearer, I drew a picture (attached).

In my picture, the purpose that R1 serves is to prevent the signal from grounding itself while still providing a reference ground return, correct?

Edit 1: The ground for L3 is in the wrong place. The ground is supposed to be connected to the bottom of L3. Also, I didn't include the power supply, which is a 9 volt battery, since it is implied.

Edit 2: I found the manual for the last super het image that I posted. I couldn't find a single mention of band switching. It only said to build the AM radio first and then build the FM radio. So is it implied that you can't switch bands on that project, you can only build the AM radio to hear the MM band and build the FM radio to hear the FM VHF band? It showed a slide switch in the parts list, but didn't say what to do with it and the slide switch looks like it goes right next to the battery. So is it implied that the slide switch turns on the power to either the AM circuit or the FM circuit and then the audio amplifer has power not affected by the slider switch? but then wouldn't that give some undesired results of hearing AM stations while in FM mode because of the crystal radio effect of any semi conductor to demodulate even with no power supply?

#### Attachments

• FM and aviation band crystal set with band select at antenna and after detector.jpg
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#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Uh, I took a peek at your latest schematic that employs BJTs but no power source. That's just for starters.

Does anyone want to take over from here? I think I've reached core meltdown!

Chris

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
No, the rules are "You must keep bashing your head on the brick wall until it feels good, or someone else wants to take over".

#### dietermoreno

Dec 30, 2012
238
Okay, so that CD drive doesn't reach core melt down, I think I'll stick with just a one band crystal set for now and ignore the workings of a multi-band super het.

#### CDRIVE

##### Hauling 10' pipe on a Trek Shift3
May 8, 2012
4,960
Steve, thanks for the offer! He'll be in good hands.

Chris

#### dietermoreno

Dec 30, 2012
238
Okay, I think I should have listened to CDDrive's advice a few posts back and just give up on VHF and FM for now. Well at least now I have a knowledge base for when I want to go back to VHF and FM. Also, when I build VHF/FM receiver, since it has to be permanent because the wires need to be as short as possible and use soldering, I will build a super regen receiver because that will provide much better results for VHF/FM than a crystal set without the disadvantages of the regen receiver. The guy on the website who built an FM crystal set, he lives in downtown Phoeniz, AZ so of course he picked up FM stations, but me living 40 miles outside of Chicago I would probably only pick up the local FM station 5 miles from my house Star 105.5 that is already picked up by my guitar as the loudest FM station (and almost always the only FM station, depending on weather conditions and time of day, on a clear night at 3AM the guitar with guitar amp connected to an audio interface can record audio from the only classical station in Chicago which is 40 miles away) that is as loud as WBBM. So FM crystal sets do work, but you have to pretty much live in an urban area 5 miles away from the transmitter to pick up any stations so FM crystal sets are not as practical as MW crystal sets and the super regen receiver is a much more practical way of receiving FM/VHF from farther away.

So now I want to work up to building an FM/VHF super regen receiver at a later time by starting with an MW regen receiver and later a MW super regen receiver. WBBM should come in ear bleeding loud with the output jack of the MW regen receiver connected to the Hi-Z input jack of the guitar amp. Star 105.5 will also likely come in ear bleeding loud all over the tuning spectrum because of slope detection since the transmitter is so close. Farther away MW stations (and likely some close FM stations too by slope detection) should come in with the MW super regen receiver.

So bottom line, if you have horses as your next door neighbors (which I do), you probably live too far away from FM transmitters in urban areas for an FM crystal set to work.

So here is my MW regen schematic design attached below. The coil form is a 18 inch x 18 inch x 16 inch moving box to make the winding easier since less coils are needed and also now I don't even need an aerial because the coil is the antenna. Just like the coil was the antenna for the MW super het receiver project that I posted earlier. I would need to make my current variable capacitor (still only partially assembled) much bigger.

To make sure that I'm doing it right, I'll continue to build the VHF variable capacitor and when its complete I'll take photos of it so you can tell me if I built it correctly, so that way I will waste less materials on messing up on a small variable capacitor rather than a huge variable capacitor.

Edit: Now that I think about it, FM crystal set is very doable for only receiving the local station 5 miles away as long as I remove the indoor antenna from the schematic entirely and put the coax feedline from the dipole aerial direct to the alligator jumper cable direct to the VHF inductor. It is not optimized because the wire connections are too long and its not soldered, but if even a guitar can pick up that station I should be able to pick up that station. Also now that I think about it, I don't think I even need to build a new huge variable capacitor, because if you look at any clock radio super het you will see that the same variable capacitor is used for both AM and FM! So I can use the same variable capacitor for both AM and FM crystal set with the only difference being the antenna/inductor (inductor is antenna for AM).

Here I drew a new schematic design for AM/FM crystal set attached below.

Edit 2: You know what, I think MW is just fine to start out with to test if everything is working before even testing the variable capacitor, and to later test the capacitor, instead of adding an unknown into the circuit (FM is the unknown).

To receive WBBM, if I can receive it on my guitar, I'm sure I don't even need a variable capacitor to receive it!

So before I build the MW crystal set with no variable capacitor, I need to buy is a minimum of (20)(18 inch x 4) of speaker wire = 1440 inches speaker wire = 120 feet speaker wire. In order to add a little bit extra for a little antenna to dangle a few feet off of the coil and for the ground connection wire to dangle a few feet and several extra feet in case I mess up, I should buy 150 feet speaker wire.

I also need to buy the alligator jumpers and the output jack.

On Amazon, total bill for MW crystal set with no variable capacitor and no aerial should be:

(1) magnet wire 22 gauge 200 feet
http://www.amazon.com/Magnet-Wire-A...1367904238&sr=8-7&keywords=magnet+wire+22+awg
$5.99 (2) alligator jumper cables, 10 pack http://www.amazon.com/SE-TL10-Clip-...=1367894939&sr=8-4&keywords=alligator+jumpers$4.59

(3) guitar output jack
http://www.amazon.com/Electric-Guit...1367895038&sr=8-7&keywords=output+jack+guitar
$2.39 Total before sales tax, shipping, and handling:$12.27

#### Attachments

92.8 KB · Views: 420
• Solderless One Transistor AM-FM Crystal Receiver.jpg
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• Antenna-Inductor Design for Solderless One Transistor AM-FM Crystal Receiver.jpg
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