96 MHz FMTX schehstic + spectrum Analyzer shot

Hi Joy,
Yes, I see that the first link on the start screen is linked directly tot he old URL. I shall correct that when I publish your circuit.

If you look at the links in all the project pages, for example your project, then the links do work fine. It is olny the start page.

BR Harry - SM0VPO

Forum is still not opening Harry from your start page.. the url has 'forumstopic' in it instead of 'forumotion'

I been using your online calculators for years. They have always been accurate for me. Nice job harry.
/D

When I built the calculaors for HHH I was also surprised how accurate they were.

Even the heatsink calculator seems to be very surprisingly accurate.

BR Harry

Haha yes but the online tool is very helpful and accurate when it comes to air core coils. 3T of 20SWG or 0.9144mm enamelled wire tightly wound on 8mm dia. air core gives 90nH. My japanese LCR meter showed the same value as well when I measured the inductance.

Interesting that I used to have two coil-winding rules:

1 - Coil winding data is a constant that varies from person to person
2 - No two people can wind the same coil given the same instructions

About 12 years ago I was asked for further information, such as the tension I used on the wire I was winding.
The original V4 "bug" used an air-cored coil. I found that a class of boyscouts used the same instructions but the frequency varied from 85MHz to over 105MHz. Thats when I started using inductors on the PCB tracks 

BR Harry - SM0VPO

My calculations showed 132nH for L5 and 63nH for L6 but it never worked satisfactorily. I came up with the values using online air core coil calculator
https://m0ukd.com/calculators/air-cored-inductor-calculator/

And coil info from your webpage

"L5 is 5 turns of 0.8mm Dia. enamelled copper wire with a 6mm inside diameter."

5T, 6mm dia, coil length = 0.8mm * 5 = 4mm

"L6 is 3 turns of 0.8mm Dia. enamelled copper wire with a 6mm inside diameter."

3T, 6mm dia, coil length = 0.8 * 3 = 2.4mm

Yes Harry. I’ll try your values in the next FMTX I built.

Hi again Joy,
I cannot remember. I shall have to look at my old notes, and this will mean a bit of searching.

From memory I believe that L5 was about 0.4 uH, and L6 was about 0.15 uH. I cannot remember the exact figures, but they were in that region.
I do recall making a rough calculation, then "dipping" them with a "magic tuning wand" (plastic knitting needle with a ferrite slug on one end, and a brass screw at the other end - ferrite increases inductance and brass lowers inductance). From this I was able to modify the coils to give the figures quoted on the homepages.

Remember also that the capacitors and PCB tracks have some inductance, so the calculations and reality will be a little erronious: "The little buggers" (electrons) cannot tell the difference between a connecting-wire and a coil-wire.

Does this help you?

Very best regards from Harry - SM0VPO

Do you happen to have the values for inductors L5 and L6 in your 150mW FMTX? I tried the matching network in my 300mW WBFM TX with 2n3866 but output was low and final efficiency wasn't good. What are their values in nH?

My calculations showing 132nH for L5 and 63nH for L6.

Ok, I understand.
PCBs for $5 ? That sounds good to me.
I do basically have the PCB making kit, but nothing has been unpacked since the move, with illnesses, renovating, etc.
I have a heap of projects to get on with, but not much time. But I really must get the light-box assembled, and then get a stand for the Dremmel.

But if you do change your mind about the prescalers then let me know. They, too, are packed somewhere but I do have a clue where they are.

VBR from Harry - SM0VPO

I am not using those prescaler chips as they aren't easy to find in Indian electronics market. The high speed flip flops are still available and they work well.

No I don’t. It’s okay. Once I am back in India I’ll duplicate the pcb of your
150mW FMTX to test the circuit. I can get it made there.. and oh yes there are cheap Chinese vendors online on eBay who can get me the pcb for $5. I totally forgot about their services. I am in US since last six months. I had moved to India in 2016 to avoid the cold weather of upstate NY and plan to return back soon.

Hi again Joy,
I am afraid I don't have PCB making facilities at the moment. But leave that with me, I cannot promise, but I will see what I can do.

Do you have the facility to drill the holes?

BR Harry

Yes replacing the 56p emitter cap with 100n cap in series with 100ohm resistor has made it super stable. I am now getting 10dB gain which I think is pretty good. I'll leave it at that.

I appreciate the offer but I would be more interested in your WBFM PCB if you have a spare one as it has coil etched on it and I want to see how stable it is. I will use 7474 flipflop as a prescaler to make the project easily duplicable in India. Two 74AC74 + one CD4040 for 750KHz reference frequency. Last time I used a single divide by four 74AC74 flipflop configuration to bring 96MHz down to 24MHz and it was then fed to one CD4060 (pin 9) to bring it down to 750KHz. It was my way of cutting back on components but when few college students in India made my project they couldn't get it to work. They wanted to set up their own campus radio station. I guess the CD4060 wasn't dividing the 24MHz clock from 7474 as it was near its upper frequency limit? That's why to make it simple I'll add another 7474 so that the CD4040 is presented with 6MHz signal only.

The RF oscillator (35mW - 2n2219A) and amplifier (300mW - 2n3866) stages are already working. I just need to add the PLL part.

Hi Joy,
That sounds fine. I was thinking that the few pf you used in the emitter was also frequency conscious and would give a higher gain at higher frequencies.

I will look forward to the new project, and I will do the same; upload it to the mirror-server for your review.

I have a couple of SMD prescalers, MB501 (8-pin 1.25mm pin spacing), divide by 64/65 or 128/129. I can send you one if you want (free, gratis and for nothing)?
With a simple PLL you can lock 96MHz VCO to a 1500kHz crystal reference oscillator (÷64).
See https://www.qsl.net/n9zia/mc145158/mb501.pdf

Very best regards from Harry - Lythall

I was able to squeeze 60mW from the amp by bypassing the 47ohm emitter resistor with a 100ohm resistor and 100n cap. It's stable and output is clean. I will submit one more project for your site, it's a 300mW FM PLL TX. I need to add the prescaler (74AC74) and other chips on the transmitter board.

Hi Joy,
Adding a pure resistive small load frequently improves linearity. If any equipment is capacitively coupled to an antenna it can also prevent a buildup of static electricity. In rain this can cause unwanted noise.

But have fun with the project and the notes. BR Harry - SM0VPO

https://ibb.co/bQR05A

Thank you Harry. I am saving your notes for future references. I wanted to shield the stages but didn’t pay any particular attention to long component leads either. The 2n3866 amplifier starts producing spurious output once its gain is increased. On the $20 SDR dongle spectrum analyzer it shows up as spikes around each harmonic of 96MHz. Will try your suggestions on the next VHF project.

With the 470ohm output resistor the transistor is cooler. Is it any good? I know that the wire antenna isn’t exactly 50ohm. The 470ohm will absorb around 5mW of RF power (2V peak). Years ago when I build a 500mW PLL FMTX I was using a wire antenna and a 220ohm 1/2W resistor at the output. It protects the output transistor in my experience when the radiating element is only a piece of wire cut to 1/4 wavelength.

Hi Joy,
You are more then welcome, but it is I who should be thanking you for sharing this project.

But I am so glad that it has been sorted out to your satisfaction. I have also learned more, and also got the impetus to update the mirror site so that and www.sm0vpo.com are now in sync (except for PDF files - no space on the free server).

If you are interested in a couple of suggestions, then I have some advice I have learned about dead-bug method of construction. I your article you mentioned the possibility of instability if the emitter is bypassed with the capacitor. One of the reasons is that the mutual coupling between the oscillator and other stages and the PA stage can cause positive feedback. Your remedy makes it clean, but you could get more out.

When using this technique for VHF I have taken to cutting some 1" (2.5cm) strips of board and making a PCB board box around each stage.

I also use a 5mm wide strip of copper down the side of the base board and use this as a supply rail. It is easy to cut 1mm gaps in the copper strip, bridge with a resistor, and put de-coupling caps to ground at the start and end of every track.

A 5mm square bit of copper board can be super-glued to the base board to make anchor points of inputs, outputs and other external connections.

If several components are hanging in mid-air then I use a 1M0 resistor, soldered vertically on the base-board, to secure the components. 1M0 is in effect an insulator in most parts of any circuit, as long as the impedance is reasonably low.

Using these ideas I found I can get better mechanical and electrical stability.

Very best regards from Harry - SM0VPO

PS - I still think that the reference to a VSWR of 9:1 looks bit of a strange value.

Thank you Harry for taking the time and energy. If I submit any future articles I'll finalize them first. The FMTX article now looks perfect.

hI jOY,
Ok, I was a bit tired when I diD that and I did a lot of cut'n-paste.
BR Harry

Thank you Harry. It looks cool. But the text is being repeated.

“60mW PA Stage

Joy has also developed a 60mW Power Amplifier stage for this project. Joy wrote:

Hello Harry,
Please include this 2n3866 based amplifier in my 96MHz FMTX article.

Output power is +18dBm or 60mW and it increased the range to 200 meters with the transmitter lying indoors. The output was loaded with a 470ohm resistor (to keep the SWR below 9:1) and a 78cm long wire antenna was connected for testing. I added a 2n3866 based class A amplifier to raise the output power to +16dBm or 40mW. It increased the range to 200 meters with the transmitter lying indoors. The output was loaded into a 470ohm resistor to protect the transistor and a 78cm long wire antenna was connected for testing. The 470ohm resistor should be removed when feeding a properly matched antenna (ground plane or Harry’s VHF J-pole antenna).”

Should be changed to:

“40mW PA Stage

Joy has also developed a 40mW Power Amplifier stage for this project. Joy wrote:

Hello Harry,
Please include this 2n3866 based amplifier in my 96MHz FMTX article.

I added a 2n3866 based class A amplifier to raise the output power to +16dBm or 40mW. It increased the range to 200 meters with the transmitter lying indoors. The output was loaded into a 470ohm resistor to protect the transistor and a 78cm long wire antenna was connected for testing. The 470ohm resistor should be removed when feeding a properly matched antenna (ground plane or Harry’s VHF J-pole antenna).”

Rest of the text should be the same.

“The 56pF emitter capacitor increased the gain and output power to 80mW but amplifier was self oscillating so it was removed. A 2n3904 can be substituted in place of 2n3866 but the output will fall to +13dBm (20mW) or so. The 56pF emitter capacitor can be used with 2n3904 without a problem. The plastic case of 2n3904 needs to be superglued to the copper clad board to keep it cool. The 2n3866 gave a cleaner output of 40mW and did not require a heatsink so I recommend the 2n3866 for its superior performance. Do not use emitter capacitor with 2n3866 to keep it stable.”

Oh, yes, add the new spectrum analyser picture.

It has been done 

Once more, please check it out and see if you are happy with it. www.sm0vpo.com
I will update this and others on altervista later.

BR Harry

dare4444 wrote:Yes Harry I want some changes. Here’s the updated text to be added under the amplifier section.
...
https://ibb.co/nbwESq

Hi Joy,
Thank you for the update. I hope to do that this afternoon. Just now I have to go out and do some shopping: the "magic refrigerator" is empty! I know t is empty, but one still has to double-check 
I will drop you  line when it is done:
- replace drawing
- replace your text

Have a nice day - Harry

Updated amplifier schematic . Please use this one.

https://ibb.co/nbwESq

Yes Harry I want some changes. Here’s the updated text to be added under the amplifier section.


"I added a 2n3866 based class A amplifier to raise the output power to +16dBm or 40mW. It increased the range to 200 meters with the transmitter lying indoors. The output was loaded into a 470ohm resistor to protect the transistor and a 78cm long wire antenna was connected for testing. The 470ohm resistor should be removed when feeding a properly matched antenna (ground plane or Harry’s VHF J-pole antenna).

The 56pF emitter capacitor increased the gain and output power to 80mW but amplifier was self oscillating so it was removed. A 2n3904 can be substituted in place of 2n3866 but the output will fall to +13dBm (20mW) or so. The 56pF emitter capacitor can be used with 2n3904 without a problem. The plastic case of 2n3904 needs to be superglued to the copper clad board to keep it cool. The 2n3866 gave a cleaner output of 40mW and did not require a heatsink so I recommend the 2n3866 for its superior performance. Do not use emitter capacitor with 2n3866 to keep it stable.

Attached is the spectrum output of the 40mW 2n3866 amplifier from 24MHz to 1.2GHz. Second and third harmonics of 96MHz are -18dBc and -25dBc respectively.

https://ibb.co/gQJ3nq

Hi Joy,
Please check out www.sm0vpo.com
The direct link to the page is http://213.114.131.21/tx/96mhzfmtx.htm

But at www.sm0vpo.com you can see either latest projects, or select the TX projects, as you requested.
Please review and let me know.

2 small comments:
1 - you referred to "SWR" but should be "VSWR" (just to be really accurate)
2 - you quoted VSWR under "9:1"

Item #2 you may want to revise. 1:1 is perfect, 1.5:1 is acceptable, 3:1 is 50% wasted power but 9:1 is almost nothing radiated. Can you please give me some new info if you want it changed? I don't change any of your test without your approval.

Now it is on the RaPI I can easily go into LINUX and do a VI edit to change data: no need for me to "ftp-put" new files (that's a pain in the bum as LINUX will only let me put them in the /tmp/ directory, then I have to use putty.exe remote connection to move them from /tmp/ to the /var/www/tx/ directory).

BR Harry

Hi Joy,
Sorry for the delay. I returned from Spain the day before yesterday and I now have got access to the raspberry-pi that is hosting www.sm0vpo.com.
It is beside my bed in the Swedish apartment and I don't have remote access to it. But that is a good thing because I only open port 80 so there can be no external access to the server. Stops hackers 

At the moment I have a WiFi hotspot open in the apartment with the SSID "Free WiFi" and no encryption (open network). It is not connected to anything at all: no internet, nada!!

1 - It attracts people looking to hack free WiFi networks (over 70 WiFi routers broadcasting in this apartment complex)
2 - I use it as a closed network to connect my android telephone camera to my computer as a high-quality, mobile webcam
3 - The attacks registered in IP-cop firewall fell by 50%
4 - At this moment there are 28 people connected to it 

I am at present updating the homepages to include your project, complete with the power amplifier. I will let you know when it is up. I will also update the sm0vpo.altervista.org mirror server with the latest projects.

BR Harry

Hello Harry,

Please include this 2n3866 based amplifier in my 96MHz FMTX article. Output power is +18dBm or 60mW and it increased the range to 200 meters with the transmitter lying indoors. The output was loaded with a 470ohm resistor (to keep the SWR below 9:1) and a 78cm long wire antenna was connected for testing.

Schematic https://ibb.co/g2rHhf

Thank you Harry for your valuable insight. I get it now. Harmonics decreased after I replaced 7 pole filter with 5 pole. 2nd harmonic is down below -30dBc and 3rd harmonic is -20dBc. The circuit board is very small and components leads are long and this explains it. I will put it inside a cookie can soon. I connected a 16v zener across the oscillator tank circuit and not from collector to ground as it gave me a slightly higher output power. I'll add a PLL with 74AC74 chip as prescaler. Here's the schematic of oscillator+PA stage. I am also using a RFC (1.5 uH) in emitter of 2n2219 to prevent RF from going into ground in order to extract  maxm power from the

Hi Joy,
There is nothing wrong with the filter circuit you showed, but there could be a problem with "mutual conductance".
Two wires in parallel are in effect transformers at vhf. Even with ugly construction you should still keep wires short, distance, and pay attention to polarisation.
Another point is the measurement method. You may be measuring the output of the filter, but are you picking up the PA stage output directly, before the filter?

Put the transmitter in a metal tin, even if it is just a biscuit tin, then measure the RF near the antenna. This will tell you if the analyser was too close to the PA stage.
If there are still harmonics then there could be coupling between wires on the board -effectively bypassing the filter.

Transmitters for commercial use are usually built in stages, each individually screened. This improves stability and reduces unwanted coupling.

Hope this advice helps. BR Harry - SM0VPO

Thank you Harry for info on power levels, Rx sensitivity, and range. I saved it. I have one more question. I added a low pass filter to my other homebrew FM transmitter. But according to the SDR dongle based spectrum Analyzer the 3rd harmonic is only 3-4dB down or so. I am attaching schematic and prototype’s pic. What’s wrong? The three inductors are 90nH in value, all copied from a commercial design here -

https://ftp.unpad.ac.id/orari/library/library-sw-hw/community-broadcasting/fm-low-pass-filter/NrgkitsFm_Com%20-%20Workshop%20--%20RF%20Low-Pass%20Filter%20For%2088%20-%20108%20Mhz%20Transmitters.htm

My PA schematic and prototype here -

https://ibb.co/bRJ1bV
https://ibb.co/kXpUVq

Hi Joy,
Just spent 40 minutes typing a long reply to you, then accidentally clicked the wong button (the X at the top-right).

Yes, the SM0VPO J-pole antenna is designed to be really easy and quick to make.
If you use a wooden pole then be sure it is dry, and stays that way. Plastic conduit tube is more tolerant. If it is for permanent installation then use chocolate-block connectors to make the actual connections (not twisted wire) and put it in a length of plastic drainpipe.

Two articles may interest you:
http://213.114.131.21/data/pathlos.htm
http://213.114.131.21/antennas/7db-colinear.htm

The things that best affect the range is:
1 - (most effective) receiver sensitivity - can increase it 200x
2 - (moderate effective) Improve the antenna - can increase range 50x
3 - (low win) transmitter power - can increase range 15x

Comms receivers can have sensitivity of -120dBm. Cheap domestic FM radio ($2.5) perhaps -80dBm, or -90dBm if you are very lucky.

A good high antenna for omni work, or a high Yagi for directing a beam to a target area from a neaby mountain, can give a surprisingly good increase in distance. A 1/4-wave (outdoors) on the roof of a car, or a 7db colinear on a huge block of flats/drone/baloon can give you a 30dB increase in signal at 1km, possibly more if it is really built up.

To increase a transmitter from 1-Watt to 500-Watts is only a 27dB improvement, but it costs a lot to achieve.

You can achieve 1,000,000 miles per watt under line-of-sight conditions.
The world record for digital comms between a wiFi dongle node and a WiFi dongle user is about 240km, 1mW.
(https://www.wired.com/2007/06/w-wifi-record-2/)
Dongles placed at focus of parabolic reflectors. Positioned on two mountains with a clear line-of-sight path.
Done in the US continent.

But have fun with the antennas. I am sure that you can learn a LOT with simple experiments.

I once built a 12db Yagi using 4x 4.5m lengths of building timber. Two 8m long booms with 12 wire elements (2.5mm house wiring cable) strung between them. I used it to receive FM stations on 107MHz. I was really messy, but once set up it worked. Supported by wooden goal-posts on the playing field. Rotated by hand (arm-strong method). Not very practical, but I just wanted to prove it worked.

BR Harry

Thanks. I just checked your J-pole antenna design. Can I construct it with 20SWG enamelled copper wire and use a plastic tube or rod to support the copper wire and hold it in place? I have never tried building this kind of antenna before. I was only using ground plane or a wire antenna for VHF.

I read the description again it's a V-pole and you instructed how to build it using a piece of wood, nails, and copper wire. 3/4 wavelength part is 240 cms long at 96MHz.. quite large but it should make an excellent antenna for outdoor use as it also has a gain of 2dB. Will try it. Looks neat and easy to build antenna and will beat a ground plane antenna anytime! I wasn't aware of this antenna design on your homepage.

Yes, I can do that. Absolutely no problems at all.

BR Harry

Hi Harry,

Is it possible to put this project under your Transmitters section? This way it will receive way more visibility than being in the visitor circuits section. It's upto you. Thanks!

Joe's, Joy's ?

Ooops... This is a 65-yrar old brain I am struggling with. Sorry! Tipping errors is one fo the reasons I asked you to review it ;-)

I will update it and put it as a public link during the coming week. I lost access to 213.114.131.21 for a few days, but I will start by adding to the mirror server http://sm0vpo.altervista.org/ , then update www.sm0vpo.com as soon as I have access to the HW.

BR Harry

The article looks excellent on your website.

“... my messageboard, supplied information for a 96MHz FM transmitter project, which I feel is of particular interest for both performance and the method of construction. Here is Joe's project including his pictures and technical description.”

Joe’s needs to be corrected to Joy’s, that’s it. All else looks good! Thank you.

Hi again Joy, 

http://213.114.131.21/temp/96mhzfmtx.htm

If you need any corrections or additions then let me know before I link it on the homepages. At the moment it is in a temporary directory, but the final link location will be "http://213.114.131.21/_visitors/index.htm#tx" on www.sm0vpo.com.

Incidentally, 4mW (+6dBm) into a modest and resonant antenna (such as my http://213.114.131.21/antennas/vpole0.htm) will give you a good 500 metres if mounted outside "in the clear" (on the roof of the house). With a good domestic receiver you should be able to get 2 km or more.

Thank you Glenn. I used your suggestion and made a JPG from a screen dump of the e-mail address.

Hope all is well.
BR Harry

Yes Harry pls include the email address.

Admin wrote:Hello Joy,
Sorry for the delay. I have been sick (again) and had a few other "urgent" panics. yesterday I had a 3D x-ray of my head ;-)
...
Can I include your e-mail address so I can direct any questions to you?

BR Harry - SM0VPO
...

Hi Harry

Hope you have recovered/will recover.

Suggestion: Please make a "screenshot" of the email-addresses and embed these (tiny) images, so it is not directly script harvestable?

best regards,

Glenn

Hello Joy,
Sorry for the delay. I have been sick (again) and had a few other "urgent" panics. yesterday I had a 3D x-ray of my head ;-)

But I have copied all your text and the pictures, so I can create the HTML file. I will upoad the finished file to the altervista mirror and give you a link when it is done.

Can I include your e-mail address so I can direct any questions to you?

BR Harry - SM0VPO



PS - I hoped to do it at work, but www.sm0vpo.forumotion.com has been blocked by my employer  
Something to do with the companies "code of ethics". Perhaps they don't want engineers to have access to a technical Q&A forum ?

Hi Harry I edited the text and made some changes in the tuning procedure. Please use the updated text. Thanks

I updated it again today. Use the latest pics and description. Thanks Harry!

Thank you very much, I have not forgotten about you.

Maj-Lis is leaving for Spain in the morning, so my evenings are rather spoken for. But I hope to put the page on the server over the weekend and I will send you a link for your review.

Thank you once again.

BR Harry - SM0VPO

Hi Harry, here are the final images and text description.

https://ibb.co/jjnMgz (24MHz to 500MHz spectrum analyzer shot. 5dB per division)

https://ibb.co/f8fk8e

https://ibb.co/gi9OuK

The crystal locked FM transmitter presented here avoids the use of a LC tuned tank circuit or complex PLL (phase locked loop) method for FM carrier generation. Instead it uses a fixed frequency crystal and IN4007 as varactor diode. The 24 MHz crystal frequency is quadrupled for a final transmit frequency of 96 MHz. The audio fidelity is surprisingly good.

Circuit and Working

The circuit is built around a low-power audio amplifier using LM386 (IC1), transistors T1 and T2 (2N3904), 24MHz crystal (XTAL1), rectifier diode IN4007 (D1), and a few other components. A low power colpitts oscillator is built around T1 along with XTAL1, biasing resistors R3, R4, and R5, and capacitors C5 and C6 for feedback. The oscillator’s frequency is set to 24MHz by XTAL1. T2 is fed directly from emitter of T1 and acts as a buffer isolating the oscillator from the antenna. T2's collector is rich in harmonic energy and this makes it an ideal frequency multiplier. The tank circuit comprising inductor L1, C7, and trimmer capacitor VC1 is tuned to four times the crystal frequency or 96MHz. The fourth harmonic of 96MHz is boosted while all the other harmonics including the fundamental frequency of 24MHz are suppressed by 21 dB or more. L2 and C11 is a series resonant circuit tuned to 72 MHz and helps trap the third harmonic to ground thereby attenuating it.

A 50-Ohms level is taken off at 1/4 turn from top of the collector tuning coil and coupled to a 78cm long wire antenna for transmission. The signal can be picked up by an FM receiver tuned to 96 MHz.

The audio amplifier built around IC1 has a gain of 20. R1 and C3 removes noise and suppresses IC1’s tendency to self-oscillate. The amplified audio from the LM386 frequency modulates the oscillator by varying the capacitance of diode D1 which is connected in series with XTAL1. No hard to find varactor diode has been used. The IN4007 gives good performance. C2 prevents RF from getting into IC1. The trasmitter accepts audio input from an external equipment, such as a computer, music player, or your mobile phone. The external volume level should be set for best audio quality without distortion.


Construction and Testing

Keep all leads short. It is a standard practice at RF and aids in stability. The prototype was built on a piece of copper clad board using ugly bug construction method. Apply 12V power from a SMPS regulated power supply. Turn on S1 and tune VC1 for voltage reading at the point marked as ‘V’. It should be around 130mV. Carefully tune the 50 pf trimmer capacitor for the highest peak reading. All unwanted harmonics are now down below -24 dBc or more. A spectrum analyser may be used to confirm that the fourth harmonic has been boosted. If you don't have access to a spectrum analyser then the correct tuning of VC1 can be confirmed by connecting a wire antenna and testing for maximum range on an handheld FM receiver by walking away from the transmitter. Correct tuning of VC1 will give maximum range on 96MHz. L1 and L2 are 90nH air core inductors.

Turn off S1 and connect a 78cm wire antenna. Use shielded cable for input audio connection. The current consumption of the transmitter is 20ma and delivers an RF power output of +6dBm (4mW) which is more than adequate to cover my two bedroom apartment with good signal strength. An external VHF amplifier may be used to boost the power output to several hundred milliwatts along with a matched ground plane or dipole antenna for 1Km range.

L1 = 3T 20SWG enamelled copper wire wound on 8mm diameter air core. Tap at 1/4 turn from V+

L2 = 3T 20SWG enamelled copper wire wound on 8mm diameter air core.

Variable trimmer capacitor = 50 pF (12-66pf variable)

100uH RFCs used are readymade moulded chokes to isolate the oscillator and buffer stage and help reduce unwanted harmonic content at the output. L1 and L2 are 2cm apart and 90 degrees to each other. This arrangement gives the best harmonic suppression. A low pass filter with a cut-off frequency of 110MHz may be used to further clean up the output signal.

Joy Mukherji
Joy226010@gmail.com

Willl add final text & schematic. Give me a day. Thanks . Yes it looks ugly but works great.

Hi Dare4444,
Yes, or course  will accept it in the visitors circuits section.
Interesting that this method of construction looks really messy, but most often out-performs a custom PCB 

Is this the final text you want in the article? Or do you want to add more explanations, purpose, reason, motivation, etc? E-mail address and name?

BR Harry