Thursday, March 6, 2014

on air

this is the third part of rm96 tranmitter:- 

Now every module is individually tested and it is time to match/ interface to each other and to be on air. Till today we have reached from dsb exciter , power supply to a small 1watt linear and then big linear stage.

Linear is readymade( homebrew) and it will be driven from dsb exciter. This linear likes the 50 ohm 1watt rf. But, homemade dsb exciter produce anywhere 2 to 5 watt RF. The ham who built this linear cautioned- not to over drive and use it with only 12volts at and not 24volts/36 volts as there will to too much power. I understood his caution. With simple dipole even with 10 to 20watts this tx will be heard all over the india.
Now my quest to be on has put me near to  the antenna and it will ultimately to your qth via ether:-

Attenuator, 3db pads:-
attenutor pad

By using this db pads, we are sure that input output impedance will 50ohms. Will save lots of headaches.

db pads, attenuator pads are the passive parts. it takes the rf and attenuates/reduces its power and pass on to other stage. they are bidirectional can be used anyway. In the web you will find online pad calculators. These are two types of pads or attenuator. One is PI- type and another is T-type. There is standard resistance values available in various tolerance, power ratings and materials. Use only carbon filament resistors. Generally they are available in less than 2watt range. No need to use high tolrance say 1%,5%. In our simple work even 10% will work, measure it with DMM. Choose the resistor with value as close as possible. Dont use wire-wound resistors. To see what material is inside the resistor,scrape one resistor's outer enamel. In the carbon resitor it will be black and if there is any wound components,anything spiral,metals discard it.
i will use the values given in J.A.G.A.W.A.N.A  blog. It is a very nice but albeit in the malay... hope he will publish it in english. He has done very good experiments and also put the clear ckts and fotos. eye candy. it is a worth to see this blog. Anyway for your db needs various online calculator are available to calculate the resistance and db values. google it......

faults of voltage regulators:-

In four different instances my voltage regulator ic have faulted. two were lm7812 became 5volts due to accidentally shorting of pins during testing some ckts. For a very brief periods these ic tolerates but it was a very real short. So, it produced only 5volts and i have to replace them. In bitx receiver input was accidentally shorted and the LM317T went bust. It happened two times. First fault has no cure unless some current limiting ckt is made or fuse is placed so that damage become minimum. For second fault there is a cure. Input short happens when there is dc voltage is on output but there is no dc voltage in input so a 1n4007 diode can be place from output(anode) to input (kethod) and problem solved. But, it cost me 4 ic. LM317t is little hard to find in my city. So, imported from metro. I think in the bitx design there should be this diode. But, anyway there were not expecting this fault as power transformer is alway connected. But, i am using the bitx-rx as qsk with my dsb tx.

Bitx- utilising only receiver portion boxed it.

bitx in a box.
I am putting this bitx into the box after a year. I got it last year. It has frequency readout and tx (not working). It gives me a very clean and clear signals. I like this for its audio quality. The homebrewer friend has really worked hard to make the xtal filter, but anyway one of the days will make me another bitx. My thanks in advance for making me a bitx. I am going slow but i have no other options. I am not keeping my date to be on air, but there are some factors which i can't help.

Another friend has promised me few xtals in the crowded section of the band i.e 14150 to 14200kc. I am waiting for these xtals otherwise people have to stretch themselves to give me a curtsey single contact. 14316kc is a way too off from the crowd. I am thanking in advance for all the friend sfor trying to be with me on 14316kc. But, after initial success ,will sure make someways to come near to the crowd from the corner.

Now is the hard test to connect the power supply switches, antenna switching and antenna with the daemon of vswr to be tamed without vswr meter.

I have waited for long long time to be on air, atleast i can wait for few more days either to say hello day after day or say finale QRT after spending lots of money.

When the Bitx was put there were few glitches i.e due to the short needed to replace the power supply ic lm317t. Then added a fuse, switch. But, audio power is little low ! boxing effect ? Marked its reading 4.272.6kc of LO freq. It means for 14.318 input signal on counter shows the 4.272kc there is constant difference of around 44kc.LO should read 4316kc. Its okey. Now, dsb-tx also tested with a dummy load and quarter wave ariel. Made final adjustment.  Put the attenuator pad power decreased but amp is stable.  I dont want more power.1watt will be nice to drive a linear.

Now time to test the linear.  Tweak the existing dipole. Then get a  SWL date from Jamanagar city, Rajkot city, bhavanagar city or from north/south , then try two way to declare either failure (QRT) or success. (QSL). LETS SEE.......

Why xtal in the age of digital electronics. I have tried lots of configurations with failure now i want to be on air quickly that's reason using the xtal tx.


Today is sunday wanted to make tx running but there was one problem, how to solve the over current up existing files in my collections. Hummm overcurrent can ruin the dream withing a minute. So, something to protect from over current ? Found answer in the SimpleCheapPA.html.  by AC2CZ/VE2 . the current sence ckt. I will modify it and re-draw the ckt.

It basically measure the current and when it exceeds (3amp), the LM7805 regulator is switched off by T1. Power to linear is cut-off and thus the linear is saved from the open-short vswr conditions.

So, the regulator also has to be little thin i.e. No, electrolytic capacitor otherwise it will create the problem as depicted above.


bitx-audio problem solved in the input pins there was a very thin stray wire shorting the input. It seems when you box all short of troubles comes forward.
current sense ckt

I ported the current sence ckt to the bc547,bc557 type common transistor as there is less current intake by these devices. Will test it today. In this also gave the resistor calculations for the  current from which the controlling starts.
For 3amp r is 0.2 ohm ,a real challenge to find it but with combo of 1.1it can be realized. 5 resistor in parallel.

11march2011 :-
0.27r a fused variety is a good one. dont use carbon resistors. modules ok. when all is okey LED LIGHTS and there is 5volt, when vcc is shorted the 5volts drops to near zero volts. this only protects the linear amp but not preventing , cutting off the shorts.

dummy load

Made a dummy load from 11 resistors each having the 560ohm resistance. 3watt carbon film variety. so load ready. It become 33watts.

12march2014 :-
I bought the computer fan. It is a fan which is placed on the computer body to force the outer air into the chasis of the computer. There is another variety of fans which is used on the CPU heatsink.

Tested the configurations. applied rf to the linear. i was hoping that there will be heat and loud noice in the nearby receiver which is placed at 25 foot far and its antenna is outside the home about 30feet away from the tx.

It was noticed that the heatsink is not becoming hot. switched off the fan. no way its cool. irf510 is a very very nasty devices. it can't be cool. so, its a problem.

removed the 3db pad. it increased the output signal in radio. connected the simple 1/4 wire.  with only tx and with linear the audio is same.....humm that means it is not amplifying at all. tried different power supply. one thing noticed the current remained same about 30 to 50 and not peaking up sympathetically with voice.

At last opened up the linear  box and presto there was the culprit. the input  BNC connector was  very loose so it was moving freely and due to that no input to the gate was reaching. so, this was the real culprit. it ruined my 2 hours preparations, my one trip to the shop and all. This coaxial wire is very thin.  Now, have to solder the coax to bnc and try again tommorow. t

13 march2014 :- Tested the linear. there is some glitch. I am not able to get it hot. there is vcc and proper bias ckt. rf drive seems is okey but not producing the rf power. hmm... where i don;t understand ? I could not find any faults ! so, waiting for the Expert advice, fone a friend option.

If this does not work, for a long time i am going to be QRT. I will not invest more money, time and energy. I have already invested the more than a rigs price over the two years. Lets pray it works as designed. i think by seeing  my frustrations other hams might also get dis-interested and loss the faith in homebrewing. but faith of friends is keep me going on.

This evening i had interesting QSO on facebook.  One was short and one was long long. thanks guys.

Thanks vu2yfd i will not quit. i have never quitted . Fact is i am a lonely ham. within 100km radious no one is there to hear even my VHF signals. Now, south indian hams understand how lucky they are, that within 100km there are lots of active hams on hf (40,20) and vhf.

arrl linear as used in RM96 which
is used to drive a push pull amp.
Main problem is, tech is there but not sharing as it looks trivial. once again fired it. with miracles its becoming hot and dummy load is also heating up. but i think it is self oscillations. tried different exiceters. i have cw tx, dsb tx and one arrl rm96 in the box. Only on audio tone it gives some output but on voice modulations rf is low. so still needs to do some tweaking and waiting for reply.

With this rm96 , 3stage linear amp i am driving the push-pull amp based on the IRF510.

PP amp takes the 1.28A at about 36volts. Heat sink needs the fan. fan takes 12 @210ma. dummy load also becomes hot which means there is some sort of rf energy coming out of the pp linear.

Another friend has a similar linear. He says it takes the peak 2amp at 24volts dc (a regulated voltage) and takes very low current during the idle. Hmmmm . Sure sign of self oscillations.  I will cure it. I know the fact that the ham who builds this linear is a very reliable person and good at his work. Please do buy from him. He has good linear pcb, know-hows and proper toroids sets in VU land.

If out of 100 only 5 does not work then may be i have the 5th. This means i need to do some exorcism to cure the ghost of murphy to be on air.
(I have to let my friend use my home-brew boxes lying ilde over years ! what else by exorcism you mean ? )

(as of 15march found out that innoncent looking co-axial cable connecting from rf transformer to the output BNC connector's connections were swapped so its braid was acting as a load and causing so much trouble. how did i figured out please dont ask. it was found after i promised to let use my boxes. he he he. How much power this linear puts i have not measured so dont know yet.


14march2014 on the coming sundays will fire it again for on air test.will use the dipole on 20 meter and with xtals- freq 14331kc, modulated tone (audio tone given to the modulator instead of voice) if other hams around 150km says ok then will try to modify / rectify further otherwise it is QRT.This much near and still not there. it happens its the  RF. (roough)

I dont think i have to further add to this page. Ya, i will not modify it except few errors and if any r&d is done it will be posted on the rf amplifier page.
Any input from friends are welcome.

17march2014 Reply came with more informations. thanks OM. It seems i have given him sleepless night also. My apologies. It was not your fault nor  mine, just it was the murphy behind me. I am restoring my first made superhet and tweaking-up to give it a new QTH.

Now i have to make it work on air. and move to 14194kc using the 10M and 4.194 Mhz xtals first dsb then ssb. later on i plan to use a vfo of 4000 to 4500kc  to cover entire band.
( as of 27 jan made the DSB modulator for 14.195kc,, see the blog )

thats it folks ! i have not invented/re-inveted the wheel.  If you can make it work then wheel is yours.


  1. hi inj nice to see all your practical experiment "try try till you succeed "
    de vu3yfd, prasad Tirupur

  2. Give me ur address and Mo number
    de vu2pep