900mW PLL FMTX

So my experience with FM range is that it depends highly on the transmitter height above ground and line of sight. Unfortunately I live in a city so the maximum (strict) line of sight distance I can get is about 3km. ~100mW covers that range. At 5km range there are one or two buildings blocking the way and it takes 1W to get a decent signal (only occasional static). This is when transmitting from the 20th floor. I have also done tests transmitting from a car, and in that case 1W only gets me 500m range even with line of sight. The receiver I'm using is a nokia phone, and last I checked it uses one of those all-in-one FM receiver ICs.

The other thing I would mention is you don't know what kind of materials are inside your wall. You might have metal wires running parallel to your taped antenna detuning it, or concrete that has high dielectric constant and high loss. Using the window isn't much better either because modern heat insulating windows all have a conductive coating. I measured a 900MHz dipole in free space and taped to the window, and while taped to the window the reflection coefficient is near 1 with resonance pushed way above to many GHz. I did this test in many apartments and even on the train, and got the same result everywhere. It seems glass windows without a coating are rare these days. I only ever got good range with the antenna outside.

Ok, I understand.
If you are fighting for a couple of dB's then you are wasting your time. 2dB here or there is but a drop in the ocean.

We will see what happens when you get the other receiver module in place ;-)

BR Harry

Yes Harry the radio is receiving other FM stations. The antenna broke so it has a 20cm telescopic antenna which is shorter than before. Is the half wave dipole antenna supposed to give a 2.1dbi gain? I kept each arm at 78cm but correct value should be 73cm (468/F in MHz) due to the velocity of propagation of electricity in wire versus electromagnetic radiation in free space. Should I form a balun with the coax just before the antenna to improve radiation pattern? Yes the frequency is right on 96MHz and the amplifier stage isn't oscillating. Second harmonic is down at -10dBc so the fundamental should be a healthy 700mW after all the harmonics and I am still unable to get a range of more than 500 meters. I received a TEA5767 FM radio module and I'll use it to test the range again.

Hi Joy,
Defective receiver? Possibly.

If you have a good analyser then be sure that your output power is on the correct frequency.
If that is ok, then check the VSWR to the antenna.
If that is ok then you can be reasonably certain the receiver is crap. Is it possible that you have had the receiver close to the transmitter and burned the front end? (does it receive other stations?).

I built a transmitter for a friend in Haverhill, uk. The output was about 25mW, and he connected it to a nice dipole antenna, then went for a drive. At about 5km he got frightened and never dared to switch the thing on again. Frightened by the thought of "the authorities" finding him and taking away his radio license.

Think about it, 1 Watt = +30dBm. A REALLY BAD receiver can receive -70dBm (70uV) - a good radio -117dBm . This gives you a link budget of 100dB. That equates to a distance of 25km at 100MHz.

If the receiver sensitivity were to be 700uV (-50dBm) then link budget would be 80dB or a distance of 2.5km. But a receiver THAT deaf would not even receive local stations.

Just a quick calculation on my fingers (and toes) :-)

BR Harry - EA/SM0VPO

Yes Harry. My apartment complex is suburban with wooden/brick apartments and trees. With half wave antenna taped to wooden wall indoors, I got 400 meters of range with my handheld radio. Looked up the output on spectrum analyzer and the 2nd harmonic is -10dBc down the fundamental so the transmitter is outputting good power but I'm not getting the range. Defective FM receiver?

Hi again Joy,
This one looks interesting. I shall put it in your page when I return to Sweden, but until then I will process it and put it on altervista for you to review.

Incidentally, have you done much in the way of checking range with these transmitters? I ask because you are quoting very low figures. With a 1/2-decent antenna, a relatively poor domestic radio and 900mW, you should be able to get several km.

I have had more than 1km with my V5 transmitter.

BR Harry - EA/SM0VPO

Hello Harry.. here is one more project.

https://ibb.co/S6WXNcT (Block diagram)

https://ibb.co/5rydgPp (Half wave dipole antenna taped to wooden wall)

https://ibb.co/4Nms7DV (RF section schematic)

https://ibb.co/NZmM2P2 (PLL section schematic)

https://ibb.co/m0KDrrW (Prototype board - ugly bug style construction on a copper clad board)





900mW PLL FMTX

I wanted to cover a radius of 500 meters and came up with this high power version of my 96MHz PLL transmitter. It is a four transistor design plus the PLL control section and gives an RF output of 900mW (no low pass filter).

It all begins in T1. It is wired as an inverted Hartley oscillator with L1, C5, C6, C9, and zener diode D1 determining the frequency. T2 is the buffer stage and it isolates the oscillator from the rest of the amplifier chain. It is fed and biased directly by T1. It delivers 5 to 10 milliwatts of RF drive to transistor T3 via C14 and R11. The 100ohm resistor makes sure that the oscillator is lightly loaded. This results in excellent audio quality. Driver transistor T3 is biased for class AB linear operation for extra gain. Biasing resistors R12, R13, and R14 set the standing current to 20ma. R15 prevents thermal runaway by limiting input current to T3. Approximately 80 milliwatts of RF power is available at collector of T3. Matching network C15, C16, C7 and L2 drive the low impedance (5 ohm) base of final stage T4. The PA stage is biased for class C operation for maximum efficiency, with T4's base grounded by R16. 80mW from the driver stage is amplified to over 800 milliwatts in T4. Its collector is matched to 50ohm output impedance with L3, C21, and C22. No trimmer capacitors have been used anywhere in the RF section. This makes tuning a breeze. Slowly compress or spread L1 with your fingers until PLL lock led D3 lights up. The voltage at point A should be around 5.4V when PLL is locked. It will lock anywhere between 4 and 7 volts but adjusting coil L1 for 5.4 volts at point A gave excellent audio quality during testing. Audio output from my smartphone was enough for full 75KHz deviation. R1, C1, R2, and C2 add 50uS pre emphasis. Increase R1 and R2 to 15K for 75uS pre emphasis.

With a dummy load or antenna connected at the output, coil L3 should be stretched for maximum voltage reading at the point marked as V. Output power was measured at 900mW on 96MHz with a 12V regulated supply. I didn’t use a low pass filter but it is highly recommended to use one to clean up the output.

The PLL section is simple. IC2 and IC3 divide the RF output by 16 down to 6MHz. It is further fed to IC4 via T5 and gets divided by 8. Pin 6 of IC4 outputs 750KHz to pin 3 of IC5. The reference frequency oscillator is built around IC6 and outputs a stable 750KHz signal to pin 14 of IC5. The two signals are compared and IC5 tries to keep the input to its pin 3 to 750KHz and it locks the oscillator transistor T1 to 96MHz. Zener diode D1 is acting as a varactor diode in the circuit and gives good performance. No hard to find varactor diode has been used. RFC4 is a readymade 1mH molded inductor and isolates the power supply to IC4, IC5, and IC6. Without RFC4 an audible tone was heard. My prototype was build on a single copper clad PCB with ugly bug construction method. Make sure oscillator coil L1 is 5cm away from the chips in the PLL section to avoid picking up audible tone from 12MHz reference oscillator. I recommend building the RF and PLL section on two separate boards and connecting the two with thin RG-174 coaxial cable or shielded wires for points A and B. Led D4 indicates RF output and that IC2, IC3, and IC4 are working properly.

I fed the output to a homemade half wave dipole antenna made with two pieces of wire each measuring 78cm in length. The ends were taped to a wall. Reasonable good signal strengths were obtained within a radius of 400 meters. Mounting the antenna outdoors at a height of 20 to 30 feet will increase the range to 1Km or more. You can use either ground plane or half wave dipole antenna with this transmitter. I stacked two pieces of TO-39 heatsink with superglue for the output transistor T4 as it gets quite hot. T3 will need a single heatsink. T2 was glued to the copper board in my prototype with superglue as it is passing 30ma of current. The output power should drop with a low pass filter connected at the output. Always use a 5 pole low pass filter if you wish to operate this TX for longer periods of time.

Coil specs:

L1 = 3T 20SWG on 8mm dia. air core spread over 6mm in length. Each lead is 1cm long.

L2 = 3T 26SWG on 8mm dia. air core

L3 = 6T 26SWG on 8mm dia. air core spread over 3.5cm in length for maximum output power and voltage reading at 'V' when load is 50ohm dummy load. Po = 0.9W

RFC 1, 2, and 3 = 70T 41SWG tightly wound on 10K 1/4 watt carbon film resistors. Value = 1.5uH

RFC 4 = 1mH readymade molded choke. Try any value between 100uH and 1mH.

Correction: Pin 8 of IC5 is connected to GND. It is not shown in the schematic.