/diy/ - Do It Yourself

[Radiofag No.73567]

Good evening gentlemen.

I think I have decided what my next "from scratch" electrical project will be.

I will be embarking on a journey to build an AM/CW transmitter. For AM mode, it will utilize high level modulation for ultimate clarity and breadth of signal.

The positive side with high level modulation is simplicity of design. Only the final RF stage needs to be modulated. The downside is that to get the final stage to modulate, a powerful modulator is required. Typically, the input numbers are close to that of the output.

Modulator design shall be that of plate modulation to keep with the fundamental idea of "maximum clarity".

Here is a block diagram of a CW transmitter. Only the final amplifier and buffer stages are keyed to produce a continuous wave modulated signal.

[Radiofag No.73571]

Here is a block diagram of an AM transmitter that utilizes high level modulation.

You can see how easily it pairs up with the CW transmitter design.

[Radiofag No.73584]

Now, the task of the "audio amp" or "modulator" is a simple one.

In the diagram I posted, it does multiple things. It takes the vibrations in the microphone, and converts them to electrical signals. It then takes those signals, and filters out unwanted frequencies. Typically the extreme lows and extreme highs inputted from the microphone.

Mixing and other audio processing can take place in this stage as well. Parts of the voice can be filtered out, boosted, or lessened to provide more of a punch on the airwaves. Finally, the processed signal is amplified from microwatts to watts.

For high level modulation, this is where the complication begins.

The processed signal needs to be boosted to somewhere between 5w to 500w depending on the power of the final amplifier in the RF stage.

Also, the signal needs to be decoupled from the RF stage via a transformer. Impedance needs to be matched as well because of variances between the tubes used in the modulator and PA stages.

This is a fairly simple modulator. It provides the very basics in signal boosting.

It uses two 807 tubes to provide most of the output to the PA. You can also see the large modulation transformer behind the tubes.

A pair of 807s would yield about 50W to the PA... good enough for about 25-50w output to the antenna from the PA.

>PA - Primary Amplifier, the final

[Radiofag No.73591]

Now, I want to keep with a classic look to the transmitter.

So my tube possibilities will be as follows:

2x807 modulator
into an 811 primary amplifier

>RCA 807 tube

[Radiofag No.73597]

>RCA 811A
For the PA.

Oorrrr option two.

2x811A modulator
into 813 primary amplifier

[Anonymous No.73601]

Nice blog and all, but...

> 5 posts and no mention of the intended use

[Radiofag No.73611]

>RCA 813 tube

Or lastly, the most powerful of the bunch.

2x813 modulator
2x813 primary amplifier

http://www.w6thw.com/audio.html
Track 29, "K6JCA 813 rig" is about how the 2 x 2 rig would sound.

As a shocking coincidence, I use the same Behringer 802 mixer/preamp that he does.

The all 813 mod/PA transmitter is capable of about 300w carrier, or about 1.2 kw peak power output..

[Radiofag No.73613]

>>73601
It is a a radio transmitter.

I am an amateur radio operator.

I will use it to talk to other amateur radio operators around the globe!

[Anonymous No.73616]

>>73613

Ok. Is it for 40m band only?
Also, I thought that amateurs preferred SSB for audio.

[Radiofag No.73631]

To keep things simple, the oscillator will be crystal controlled at first. It will be designed for single band, 75m phone first, although I may add 160m and 40m at a later date.

The popular AM frequencies are:
1.888 mhz (160m)
3.885 mhz (80/75m)
7.290 mhz (40m)

Adding a direct digital synthesis variable frequency oscillator (or DDS-VFO for short) at a later date is possible as well.

[Radiofag No.73637]

>>73616
SSB is typically used for most communications, but I like vintage gear. There are many like me. There are quite a few AM-only nets out there.

AM rigs are more easily built by a hobbyist level electronics guy like me.

For simplicity, it will be fixed on 3.885 mhz (80/75m band) at first, but I will change that at a later date and add a little more flexibility.

[Radiofag No.73641]

And now, QRO tubes (high power) and amplifiers!

[Radiofag No.73644]

[Radiofag No.73648]

[Radiofag No.73646]

[Radiofag No.73651]

Also, questions related to antenna theory, RF propagation, transmitter/receiver design, ham radio, cb radio, and whathaveyou are welcome as well.

[Anonymous No.73654]

>>73641
>>73644
>>73646
>>73648
>>73651
What's with the huge copper heatsinks?

[Radiofag No.73661]

>>73654
They're Soviet.

Radar tubes.

Kinda like tubes likely used for the amplification of the signal for this baby

>DUGA-3, long-range over-the-horizon radar array

[Radiofag No.73669]

>>73661
Slightly better picture, from the top of the array.

[truckerfag !!xXmBuWkIT3n No.73689]

Hey radiofag. Thanks for the suggestion on the w2000 antenna. Can't hear for shit behind me, but I've got a 6 mile range to the front of the truck now.

[Anonymous No.73699]

>>73669
>>73661
Brain Scorcher?

[Radiofag No.73727]

>>73689
Co-phased or single?

>>73699
As it is known as in STALKER!

[truckerfag !!xXmBuWkIT3n No.73801]

>>73727
Single.

[Radiofag No.73839]

>>73801
Hah! I can tell you why.

If 0 degrees if facing forward, you're throwing most of your signal forward at a 330 degree angle towards oncoming traffic.

This is because the antenna is located towards the front left of the ground plane, as opposed to the center.

You're hearing little behind you because the rearward angle of your signal 160 degrees, so off into the fields somewhere.