TT’s Jottings-Blog of VU2SWX

by R. Jayaraman (VU2JN)

This note describes a compact Antenna Tuning Unit (ATU) that I assembled recently. It is capable of handling the full 100-watt output of my FT-840 transceiver.

For efficient radiation of the RF power otput of a HF transmitter, its output impedance, the characteristic impedance of the transmission line (usually coaxial cable), and the radiation resistance of the antenna should all be the same. Over the years, this standard impedance has evolved as 50 ohms for communication equipment — applicable to RF transmitters, receivers, coaxial cables and even the Standing Wave Ratio bridge (SWR bridge). The departure of the impedance seen at the transmitter output from this standard value is shown by the SWR bridge. A SWR of 1.0 indicates an impedance of 50 ohms resistive.

An antenna, when cut for the band of operation, is said to be a resonant antenna. At the antenna end of the transmission line, the RF impedance of a resonant antenna is a pure resistance known as its ‘radiation resistance’ whose value, being different for different types of antennas, is not always close to 50 ohms. Moreover, when it is not 50 ohms, the coaxial cable might transform this impedance to some other value at its transceiver end. So, even a resonant antenna might be seen by the transceiver as having a SWR higher than 1.0. Because of the difficulty of putting up an antenna for each band of operation, we are often constrained to operate using a non-resonant antenna, which appears as a complex impedance made up of its ‘radiation resistance’ plus a significant capacitive or inductive reactance. Both these components vary with the frequency of operation.

Present-day solid-state transceivers, which have protective circuits that sense the SWR seen by the rig, would not load antennas that show a high SWR. Moreover, these transceivers make use of a bank of bandpass filters near the antenna terminals that would provide the required bandpass characteristic only when seeing a 50 ohms impedance. Therefore, these rigs need an ATU to work with antennas that show even a moderately high SWR exceeding 1.3.

In many situations, we rely on an ATU to enable us to operate with a ‘short antenna’, i.e. one whose resonant frequency is higher than the frequency of operation. A short antenna appears to the transmitter as a complex impedance in which the resistive component or ‘radiation resistance’ is much lower than that of a resonant antenna, whereas the capacitive reactance is substantial and dominates over the resistive component. It is the job of the ATU to transform the complex antenna impedance to 50 ohms resistive as seen by the transceiver. However, when we operate with a short antenna in this manner, a part of the transmitted power is wasted (1) as ‘line radiation’ from the coaxial line, and (2) as increased ‘resistive losses’ in the coax and the antenna due to the higher RF currents needed to radiate power from the lower ‘radiation resistance’ of the short antenna. The ‘resistive losses’ occur due to real resistance of the conductors at RF (which again is different from the resistance at DC), whereas the ‘radiation resistance’ of the antenna is a virtual resistance which can be calculated theoretically for any antenna (and frequency of operation), and which governs the RF power radiated by the antenna.

Long ago, when I commissioned my ham radio equipment, I was keen on building a good ATU. The general belief then was that a Rotary inductor was an essential component of the ATU. Since I couldn’t locate either a rotary inductor or a 12-position antenna switch for use with a tapped inductor, my ATU project did not take off and, after a while, I lost interest in it. I was QRV only on the ham bands for which I had antennas.

About a year ago, my friend Salim, VU2LID / N8LI, who works in USA and visits India often, suggested that I try operating on 80 metres, and loaned me his SPC Transmatch, which enabled me to tune my 40-metre dipole on 80 metres. Because of the success of the 80-metre operation, my interest in ATU’s got revived. The elusive 25 uH rotary inductor was also finally located. However, the cumbersome size of the rotary inductor and its dial drive put me off, and I started surfing the internet for circuits of compact ATU’s that didn’t need a rotary inductor.

I was able to locate some articles on the Z-match ATU authored by G3VGR, VK5BR and others. Their circuit used a tapped coil (toroidal or air-core) with a link feeding the antenna (Fig.1). I tried this circuit and found that its tuning range was very limited, necessitating tricky adjustment of the total turns and taps of the coil. Also, I didn’t like the link coupling for RF power transfer. So I continued my search, and finally located the article by XS4ALL on the elegant Fri-match ATU, originally developed by PA0FRI. Fig.2 shows the circuit diagram of the Fri-match ATU. This ATU uses a single tapped coil (toroidal or air-core), which couples directly to the antenna. An interesting feature of the Fri-match ATU is that the input and output of the ATU can be interchanged. The conjugate configuration is said to work better in some situations.

 

A brief discussion on the use of BC-type air-variable tuning condensers for RF power transfer would be appropriate here. In many published articles, the suggested plate spacing for the variable condenser of a 1 kW ATU is around 2 mm. For RF power levels of upto about 200 watts, such wide-spaced condensers are unnecessary, especially when low-impedance antennas are used. I always test BC-type variable condensers before using them in my ham projects, by connecting a 230-volt 10-watt bulb in series with the variable condenser, then applying 230-volt AC to the combination, and turning the condenser knob to and fro to check for arcing. Fig.3 shows the test setup. A good-quality 2-gang 500-pF BC-type air-variable tuning condenser (Polar, Sanyo etc.) would generally pass this test without any arcing. This means the condenser can handle 230 volts RF, which appears to be quite adequate. In fact, it is not the plate spacing of the condenser that appears to be critical here, but rather the RF current-carrying capacity of the wiper of the condenser. At any rate, a good BC-type air-variable condenser, tested before use and having a clean wiper, should be quite adequate for RF power levels of upto 200 watts.

I had with me 2 Nos. of 1.56 in. o.d. toroids of unknown permeability characteristics, which were found to be good for HF. I stacked the 2 toroids, wound teflon tape over them, and then wound the coil using 14-gauge enamelled copper wire. Winding the toroidal coil was a real pain. Silver-plated multi-strand soft copper wire with teflon insulation would have made the job easier, but it is not available here. The number of turns needed for the toroidal coil depends on the core area and permeability of the core. Suggested number of turns is 15, 20 or 25. I used 20 (n) turns with taps at 4 (n/5), 8 (2n/5) and 12 (3n/5) turns.

 

 

To make a long story short, my Fri-match ATU was completed in March 2011, nearly half a century after I first thought of building an ATU! Fig. 4 shows a photo of this ATU. It has just 2 controls, and no rotary inductor. It outperforms the conventional Z-match with regard to ease of tuning and tuning range, and is almost as good as the SPC Transmatch. And interestingly, so long as the Fri-match ATU is able to match an antenna within its tuning range, it is able to bring down the SWR to exactly 1.0. This is something that I had not expected from a 2-knob ATU that is free of the burden of a variable inductor!

No reduction drives are used in this ATU. Though the tuning of the condensers is very sharp, it is manageable, even for a person aged 75 years! An analog SWR bridge is needed for tune-up. A point to be kept in mind is that, if one of the condensers is very much off-tune, tuning the other condenser would not produce any dip in the reflected power. Therefore, in the absence of calibrated dials, visual monitoring of the condensers is necessary. The body of one of the condensers has a RF potential but, since it is tied to the transmitter output, there is no hand-capacitance effect.

The Fri-match ATU sits to my right near the front edge of the operating table, not far away from the FT-840 transceiver. From the antenna switch, a 70-ft. length of RG-223 coax feeds a 40-metre dipole antenna, and a 50-ft. length of RG-213 coax feeds a HY-GAIN 12AVQ 3-band ground-plane antenna. The ATU enables me to use the 40-metre dipole on 20, 40 and 80 metres, and the 12AVQ ground-plane on 10, 15, 20 and 40 metres — all with a SWR of 1.0 as seen by the transceiver. So much so, the ATU is useful even when a resonant antenna is used for the band of operation. On 20 metres and the higher bands, I normally use the 12AVQ ground-plane. The only time I operate with a non-resonant antenna is when I use my 40-metre dipole on 80 metres. Signal reports then indicate that I am roughly 1 S-point weaker than similar stations using a 80-metre dipole. That’s not bad, and I am quite happy with the performance of the ATU.

I recommend this ATU to all hams. When an ATU is available, we can fabricate a dipole, ground-plane or any other antenna simply to the dimensions suggested by theory, and dispense with the trimming of the antenna. In many situations, trimming of the antenna to lower the SWR is unscientific, because the problem is not in the antenna, but elsewhere! It is better to rely on the ATU to take care of the fine tuning of a resonant antenna.

– VU2JN.

(  OM Jayaraman VU2JN has kindly  permitted me to put this note on tis blog. Many thanks  to him for sharing his experiences. VU2SWX. )

vu2swx Ham Radio

I have been  playing with the ISDR software defined radio from OM  VU2DEV ,  Ramaprabhu ( from Bangalore )for the past one week.  ISDR is a  low cost SDR for  40m band.  The radio is  performing wonderfully.    I bought 2  assembled kits from DEV and  I recommend the kits  to any one who wants  to  have a taste of SDR technology.  The  fully assembled radio is available with VU2DEV at Rs 750 ( US $15 approximately) and comes packed in a rugged metalic enclosure and with necessary  audio cable.  ( Email vu2dev  at yahoo.com for  availability,  exact pricing and shipping info. ).

The following pictures   show the professional quality ISDR  .

OM VU2DEV has also sent  me the schematic  diagram and component values   and  permitted me to post the same on my blog.

The block diagram of the radio is shown below. (Click on the picture for a larger view)

At the heart of  ISDR is a 28.224Mhz  crystal which is used to create stable  oscillations. This frequency is divided by 4  using two 74HC74 dual flip flops to produce  two  90 degree out of phase  local oscillator signals for the radio.

On the RF side signals from the antenna are  filtered for the 40m band and some  RF amplification is given.  The RF signal is then mixed with  local oscillator signal using two diode ring mixers.  The resultant  signals  are   sent to a simple low pass filter to get rid of unnecessary  mixer products and then amplified.  The   two quadrature signals are fed to the mic input of  a PC sound card.  ( You need a sound card with stereo input).

The sound card is used as  analog to digital converter. The normal sound card found on your PC will have a sampling rate of 44khz ,with  16 bit accuracy.  The sampled signals are  processed digitally inside the PC using SDR software and you can hear the radio via PC speakers.   Your tuning range will be half of the PC sound card’s sampling rate.  So with  44Khz card , you can tune from 7034Khz to 7078Khz. If you have a professional quality sound card,  your milage with SDR will be better.

The circuit diagram and  component values are  given below.

If  you want to look at the circuit in finer detail,  download this  pdf file.   The info sheet provided by vu2dev is here .  The component values for the circuit can be found here.  T

I have successfully used  Rocky,  kgksdr and winrad on my PC running Windows XP ( , I had to install it at last  ).  I am yet to test the SDR on linux and will update as and when I am able to do so.

My Intel  G35 mother board seems to  supports ( I am suspect )   96khz sampling via the onboard Realtech card.  Hence , I am able to tune almost the entire 40m band. Here is a recording from the  radio using  Rocky.

Update  22-9-2011
OM VU2DEV   at his morning QTH M/s Micronova Impex Pvt Limited  is making  a large number of   kits including ISDR for the benefit of the community.  You may contact them directly.  The contact email id is <vu2dev@yahoo.com>.

I have used his  JOTA transceiver  and reviver . Both are valuable additions to your  shack. He has also provided me with PCBs of his new 72 watt linear, I am planning to build them during my next vacation

 

 

vu2swx Ham Radio

Here are photographs of my shack. I have built several equipments and  circuits over the years. These pictures show the status of my shack as on September 2010.  I took them last week on a Nikon CoolPix L21 .

vu2swx Ham Radio Ham Radio

I was playing with  my bitx build yesterday. Specifically, I tried to add CW  to the rig.   Original bitx20 is a SSB rig.  I my mod was inspired from PY2OHH bitx40.  I  wired  up a 10mhz crystal oscillator as shown below.

The idea is to inject 10mhz ( approximately) at the  beginning of transmitter IF amplifier. This will in turn produce 14 mhz sine wave at the output.  The receiver part is unmodified.  See the diagram below.  Connect  the point marked X to the point x .

Adjust the  10Mhz oscillator to  around 700khz above the  BFO frequency. I added a small inductor (10 micro henry) in series with the crystal to adjust the frequency near BFO,.It is not shown on the figure above.

vu2swx Ham Radio bitx, bitx qrp, bitx40

Several friends have asked me to explain the procedure and formalities of becoming a ham radio operator in India. Unlike many of the western countries , the active population of amateur radio operators in India is very minuscule. The major reason for this is the cumbersome procedure and the lack of interest from concerned authorities.

Obtaining a license can be a tiresome process. The general algorithm for obtaining a license is some thing like this.

1) Decide whether you are really fascinated by the hobby.

2) Get hold of the application forms and fill them out.

3) Send them to the nearest monitoring station

4) After 2 to 4 months you will get get an intimation from the monitoring station, asking you to appear for the ASOC examination.

5) Write the examination and forget about it. If you are successful, you will get another  intimation from the monitoring station  within 2 to 3 months.

6) Get yourself into an infinite loop. This will allow the government to verify about your antecedents

7) If you are lucky, the Ministry of communication will interrupt you to remit the license fee ( you are still in the loop )

8) Once you remit the fee, you may get a license in another 2 ( may be more) months.

By this time any reasonable Indian will have lost his interest in the hobby.

I will now attempt to give more information on tackling the algorithm.

Step 1

This is the most important step in the process. Read about the pros and cons of becoming a ham. Whether you will have the time ( and money) to build and operate a station. Interact with a nearby ham about the virtues. If you are a geek, you have a better chance.

There are several website giving information on ham radio. Look  around them. You can start with  this and this.

Get hold of a decent radio ( ordinary broadcast band radio with short wave band.) . Most of the amateur operators in India work on 40M and 20M. You can easily tune to 40M on an ordinary radio. The 40m band is from 7.0 to 7. 1.Mhz. Most of the radios will have 41M marked on their dial. So our 40M band is just below that.

Build a simple VFO and place it near the radio. When you turn the  tuning  capacitor on the VFO , you will get a sharp whistle on the radio. Now,attach  a long wire to the telescopic Ariel  of the radio. Make the length about 10m and  hang it horizontally. Tune around 40m band.

The circuit of a simple 40m VFO is given here.

Most of the amateur activity on 40M band is around 7am to 8.30am  IST  on weekdays. On Sundays, you can find some stations through out the day. However,  better band conditions prevail during mornings.

Listen for some days and see whether you like to be a ham.  A  good ham station can be built at a cost of say   Rs 1000.  You  can alternately purchase equipment costing lot of money. For building a station you need to put in lot of time and effort. Think well before starting out.

Step 2

For me this was the most difficult step. I will give you the details.

In India. there are four kinds of amateur licenses. They are:

1) Advanced grade

2) Grade 1

3) Grade 2

4) Grade 2 (restricted)

These licenses are issued by the Wireless Planning and Coordination section(WPC) of Ministry of Communications Government of India.( They have a website but seems to be down for some time ) For obtaining the license, you must appear for an examination conducted by WPC. The examination will cover topics in basic electronics, radio regulations and Morse code. This examination is called Amateur Station Operator’s Certificate (ASOC) examination. The rules regarding amateur radio is here. The syllabus for the examination is also available along with the rules.

Depending on your license, the power output and frequencies that  you can use vary. For reasonable operations on the air, you need at least a grade 2 license.

The application forms for the examination is here. You have to fill out the form. Against item no 9 (Particulars of apparatus used), you can write “home brewed”. You  need to fill in 4 copies of personal details . and   4 photographs.

Take a Demand draft (DD) for the required amount. The draft must be purchased from State Bank of India.( This is very specific,no other bank acceptable. )  You can apply simultaneously for grade 1 grade 2 and grade 2 restricted. It is a good idea too. So purchase a draft for Rs 40. in favour of " PAY AND ACCOUNTS OFFICER (HQ), Department of Telecommunication " payable at Service Branch, New Delhi .  The guy at the bank counter looked at me scornfully when he saw my application for  demand draft. The DD commission was around RS 25.  He also warned me that I may not be able to cancel the DD later on.

There are few attachments to be submitted .

a) Nationality certificate . The format of nationality certificate is here. The certificate must be from a  Gazetted officer of the state or central government or from the Principal of a school/college.

b) Personal details 4 copies.

c) photographs (4 nos)

d)(Optional)  If you have a B Tech in Electronics and Communication Engineering, you can furnish a copy of the certificate. You will get exception from  part 1 of the examination.

Step 3

Once you have filled in the application and collected all certificates , send the application to the nearest wireless monitoring station. The  list of monitoring station are available here. Please note that your DD is payable at New Delhi, even though you are sending the documents to  a local monitoring station.

The examination will be conducted once in three months in most of the monitoring stations. They will  intimate you the date of examination approximately 2 weeks before the examination.

Step 4

Preparing for the examination

Now you have to prepare for the examination. There was a book named ABC’s of Amateur Radio in India published by EFY. I think it is almost out of print. You can download the book ” A Guide to ham radio ” from Vigyan prasar.

Learning Morse code

There are several software’s available for learning Morse code. Links to some of them are listed below.

There a good tutorial CD published by ARRL. You can buy it here.

Alternately, you can download lessons teaching Morse code in KOCH method from here.

PC  based tutorials are available on the Internet. You can try cwcp on GNU/Linux systems.

Learning Morse code can be difficult for some people. But if you practice it consistently , it can be mastered in 3 weeks time. The Morse code test has 2 parts. Morse code sending and receiving. Sending part is pretty easy .

Step 5

The actual examination is a silly affair. There will one only one or two candidates  present. The WPC officials conducting the examinations are  generally helpful.

See this post for some sample question papers.

Step 6  7 and 8

You don’t have much to do other than  waiting for a letter from WPC.

There will be a police verification.  But it is done in  a clandestine manner. Don’t bother about it, if you have a clean record.

Once you get it remit the fee  and get hold of a call sign and enjoy your hobby.

Probably, you will be 2 to 3 years older by this time.

vu2swx Ham Radio, Living in India