Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Mar Thu 21, 2019 8:45 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 34 posts ]  Go to page 1, 2  Next
Author Message
 Post subject: AM transmitter question
PostPosted: Mar Fri 15, 2019 1:14 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 19654
Location: Warner Robins, GA
Is it legal to build a small AM transmitter using a 7.2MHz TCXO with the transmitter only transmitting a few feet?

Have a radio that only tunes from 6-9.1MHz and want to send my own signal to it.


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 1:16 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 30486
Location: SoCal, 91387
Why would you worry about legality if it's entirely within your own home? Who could report you?

_________________
\\\\\\\\\\\\\\\\\\\\\\\\He Who Dies With The Most Radios Wins//////////////////


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 1:18 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 19654
Location: Warner Robins, GA
Because I prefer to do it right.

I also have a ham next door.

7.2Mhz is within an amateur band though so I have to think about that as well.


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 2:22 am 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 167
No it's not legal. I have a circuit for 7.2MHz AM transmitter using one colpitts oscillator and LM386 as modulator. Range should be a few feet without antenna.


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 3:04 am 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 19654
Location: Warner Robins, GA
Figured it wasn't legal.

Are there any legal frequencies between 6-9.1MHz?

I'm talking about no more than a few feet even if it means I have the transmitter right next to the radio.

The purpose is I want to use that radio for more than receiving the few broadcasts left between 6-9.1MHz as I have it connected to a decent tube amp and a decent speaker which should sound good playing big band and swing.

I forget how, but several years ago I used one of the LM-20 XTAL calibrated frequency indicators and an audio transformer and somehow got an AM signal which could be received quite well. Didn't transmit very far though.


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 11:52 am 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 167
Transmit outside the ham band (7.0 - 7.3mhz) and you should be fine.


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 12:29 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 167
6.4 mhz would be okay for transmission. Crystals are also easily available for this frequency.


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 1:00 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 19654
Location: Warner Robins, GA
The issue would be finding a TCXO that can handle a varying voltage then I could modulate it with a LM-386 or perhaps a transformer.


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 1:26 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 167
https://ibb.co/BnrDJ05"

Use this circuit. Frequency is fixed by the XTAL. It works well with 6.4 - 9 MHz crystals. It will transmit 5 feet without an antenna connected.


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 3:02 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 167
One mistake in schematic.. replace the 330 ohm emitter resistor with two 100 ohms in parallel for a 50ohm total. Any lower and the oscillator doesn't oscillate. Lowering the emitter resistor to 50ohm gives good modulation characteristics.


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 3:12 pm 
Member

Joined: Mar Thu 01, 2018 1:30 am
Posts: 167
https://docdro.id/4xPBagh

My AM shortwave transmitter article


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 5:03 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 19654
Location: Warner Robins, GA
Cool I may have to check into that.


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 5:47 pm 
Member
User avatar

Joined: Aug Tue 28, 2018 9:22 pm
Posts: 433
Location: Sanford Fla 32771
Quote:
No it's not legal.

My signal generators (test equip) will transmit a few feet. Can also be externally modulated.

_________________
Paul of Florida


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 8:32 pm 
Member

Joined: Nov Mon 06, 2017 2:35 pm
Posts: 39
Location: Texas, U.S.A.
...
Transistor modulator circuit proposal for modulating crystal oscillator Q3.
The ratio of the R4 R3 divider may have to be adjusted a bit to compensate for Q1 base current, to get the desired 1.7 V dc at Q1 base.

Attachment:
Transistor_Modulator_Topology.jpg
Transistor_Modulator_Topology.jpg [ 139.18 KiB | Viewed 168 times ]

WB5HDF


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 9:30 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 19654
Location: Warner Robins, GA
So in the last circuit where does the xtal connect?

Also is it possible to use a TCXO or is that what its designed for?

If so can I use a higher frequency TCXO and a counter circuit to divide the frequency down to a usable frequency?


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 9:37 pm 
Member

Joined: Apr Thu 14, 2016 8:25 pm
Posts: 436
Location: pensacola fl
Hi TR here is the fcc rule concerning what you wish to do. It is under part 15.


Electronic Code of Federal Regulations

e-CFR data is current as of March 7, 2019

Title 47 → Chapter I → Subchapter A → Part 15 → Subpart C → §15.223

Browse Previous | Browse Next

Title 47: Telecommunication
PART 15—RADIO FREQUENCY DEVICES
Subpart C—Intentional Radiators
§15.223 Operation in the band 1.705-10 MHz.

(a) The field strength of any emission within the band 1.705-10.0 MHz shall not exceed 100 microvolts/meter at a distance of 30 meters. However, if the bandwidth of the emission is less than 10% of the center frequency, the field strength shall not exceed 15 microvolts/meter or (the bandwidth of the device in kHz) divided by (the center frequency of the device in MHz) microvolts/meter at a distance of 30 meters, whichever is the higher level. For the purposes of this section, bandwidth is determined at the points 6 dB down from the modulated carrier. The emission limits in this paragraph are based on measurement instrumentation employing an average detector. The provisions in §15.35(b) for limiting peak emissions apply.

(b) The field strength of emissions outside of the band 1.705-10.0 MHz shall not exceed the general radiated emission limits in §15.209.


here is a link to the fcc
https://www.ecfr.gov/cgi-bin/text-idx?S ... ain_02.tpl

this should help out as to legality of what you wish to do.


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 9:49 pm 
Member
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 19654
Location: Warner Robins, GA
So basically the power would need to be kept real low as in much lower than what is used for part 15 AM transmitters, right?

I may just fix up one of my LM-20 frequency meters as I'm pretty sure that produces a real low output.

I would add a modulation circuit to it somehow though. Wouldn't care about output tuning either as I am not trying for distance.


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 10:37 pm 
Member

Joined: Apr Thu 14, 2016 8:25 pm
Posts: 436
Location: pensacola fl
Well they look at signal strength not power so if you had an antenna that was say 2 feet long and a 100 mw transmitter you would have a weak signal compared to say a 10 foot antenna and the same transmitter. If it were me I would keep the antenna at about 2 feet at that frequency and I would stay off the ham band to keep peace in the neighborhood. I would go to 7.5 to 9 mhz as your radio stops at 9 mhz.


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Fri 15, 2019 10:41 pm 
Member

Joined: Nov Mon 06, 2017 2:35 pm
Posts: 39
Location: Texas, U.S.A.
TR,
1. Q3 in the schematic I posted corresponds to the oscillator transistor in Dare4444's designs (the transistor that has the crystal connected to it).

2. Yes, an oscillator module of higher frequency can be used and divided to the desired frequency using logic IC's. A suitable filter circuit would be needed for the output frequency from the divider.

3. Building your own transmitter is perfectly legal in the United States.

4. If the TCXO module is capable of being amplitude modulated by varying dc voltage around 6 volts dc at one of its pins, that pin can be connected in my schematic instead of Q3's collector.

5. 30 meters is 98.4 feet. If the signal is noisy or not detected at that range by a handheld shortwave receiver with whip antenna, the field intensity is far below 15 microvolts per meter.

WB5HDF


Top
 Profile  
 
 Post subject: Re: AM transmitter question
PostPosted: Mar Sat 16, 2019 1:14 am 
New Member
User avatar

Joined: Feb Fri 15, 2019 11:19 pm
Posts: 9
Tube Radio wrote:
... can I use a higher frequency TCXO and a counter circuit to divide the frequency down to a usable frequency?

You would have to devise a modulator for the output of the divider chain in order to make that work.


Top
 Profile  
 
Post New Topic Post Reply  [ 34 posts ]  Go to page 1, 2  Next

All times are UTC [ DST ]


Who is online

Users browsing this forum: Mike Toon and 1 guest



Search for:
Jump to:  


























Privacy Policy :: Powered by phpBB