This is my very first scratch built electronic project. A 10mW 88 to 108 MHz amplifier for my low power fm transmitter. yes i still need to add a B+ choke and ground those unused solder pads to minimize stray capacitance and i might add one more pole to the LPF, then it's installation and off to the SA for the LPF to be tuned and then adjusted to comply with 250uV/m @ 3m.
for the armachair lawyers and kilocycle cops no i will not be running it full bore. it will be adjusted using my SA and compliance dipole for 250uv/m @ 3m from radiating antenna.
Robert,
You have kept the component leads at minimal length which is good practice for VHF circuitry. It appears the pads were created using a Dremel type tool which is a very good way to quickly and inexpensively prototype circuits. One of my recent builds was done in this manner as was the prototype for my AM transmitter.
Thanks for sharing the photo and let us know how it works.
Neil
Isn't it great, Robert?
It's YOURS and you made it!
I've built a few things here and
there over many many years.
I don't know if this applies to
your RF amp - but I always enjoy
putting a big huge LED and ON/OFF
power switch on the front of whatever
it is.
And don't worry if it doesn't work quite
right. A lot of the fun (for me anyway)
is trouble shooting it - or optimizing
the circuit so it will work right.
I started building electronics when I
was about 15. I'm 59 now - and it
takes me right back to when I was young.
Oh - I just looked back at the picture you
sent along with your note. Gee - it looks
really good! It looks like you've done
everything right.
Bruce, DOGRADIO
Very Kewel!! What else can I say?
Frankly I like the approach of having a controllable RF output stage with Part 15 FM.
The Ramsey 25B has one, and it really helps with compensating for antenna and other circumstances.
As recently reported, I learned how to dismantle the C.Crane FM transmitter to solder a broken antenna connection, and it's already come loose again.
While I'm at it I'd like to add Robert's output stage and a proper RF connector.
The C.Crane literature suggests adding length to the tiny telescoping antenna by using a clip and wire, and indeed the spectrum analyzer shows this adds very small improvement.
Might as well have it all.
here is the amp fully complete...
It is true joy to gaze upon electronic circuits.
We could have a circuit website in 3D.
What will come next, Robert?
A little cabinet?
How will the exciter be connected to the RF output..... cable connection?
i have 50ft of rg174 and a 1ru rack mount project box. i will be putting the decade from AR in the cabinet as a driver stage and i have a +/- dual rail 15v switching supply from a satellite receiver to power everything and it will have an SO239 on the rear. so it will be a 10mW exciter to feed a dipole to which i will adjust the whole setup to 250uV/m @ 3m about 60dbu (on SA scale) @ 3m. since this will put the antenna outside my range will be slightly better without the window screens inpeding the current indoor rig. i will likely get a Ramsey TM100 dipole.
Here is some info Robert, on the tm 100 clone i built from a diy tutorial online.
http://www.part15.us/comment/17384
It works rather well and is cheaper.
i just said f^^k it, ordered a TM-100. i know it's more expensive than home building but it comes with all the parts and they are pre cut and fit together. no headaches or wasted time.
for the same reason.
I had built TONS of different antennas
for the ham bands. But I needed a 40
meter dipole and didn't have time to
build it. It's a good antenna. But for
me, there is a lot of joy in building antennas,
so I only did it that one time. Still - I'm glad
I have it in case I need it.
Bruce, DOGRADIO
something as simple as a single band HF dipole i would just build as well.
Back in the days when I had a lot of
ham radio tube HF transmitters, I would
tune them into a dummy load. And the
dummy load was always a light bulb (then)
because to me that was really really cool.
Dip the plate and blamo, that light bulb is
glowing really bright.
BUT is a light bulb 50 ohms? Well - uh -
I guess not. So I was tuning transmitters
up incorrectly. I would get bad SWR readings
on the air. Then I would try to change the
length of the dipole to compensate which
would add to the confusion. Then things
would be worse. That's when I bought
that 40 meter dipole. I figured there would
be nothing wrong with it. One less messed up
thing in the equation. Also, there was the
matter of my eyesight.
Anyway - I use solid 50 ohm dummy loads now.
Not as fun to look at, but dependable and correct.
(I hope.)
Bruce, DOGRADIO
Does an incandescent light bulb have a fixed impedance?
If so, you could include it with the proper resistor to equal 50-ohms and maybe... just maybe.
No, a light bulb does not have a constant impedance primarily because its resistance depends on its temperature. Higher temperature equals higher resistance.
But, this does not keep them from being useful. When I first acquired my ham low band transmitter I did a calculation based on the expected output power and determined that a 150 watt flood light would provide a 50 ohm load when illuminated. It worked well enough that the checkout was successful and, as Bruce mentioned, it was fun to watch as it was tuned.
Neil