Dxers Unlimited
Dxers Unlimited's weekend edition for 14-15 June 2003

By Arnie Coro

radio amateur CO2KK




Hi amigos radioaficionados and TV DX enthusiasts , welcome to the weekend edition of your favorite
radio hobby program, coming to you via short wave and via the world wide web... I am Arnie Coro,
radio amateur CO2KK your friend here in Havana and host of this twice weekly show , RHC's long
standing contribution to the promotion and development of the radio hobby. Here is now item one:
Solar disturbances continued this week and we saw the peak of this solar rotation sending the number
of sunspots way up, while at the same time solar flares and coronal mass ejection's combined to
produce really very poor HF propagation conditions. Item two: What to do when HF propagation hits
bottom ?

Stay tuned for the answer to this question... The question again : What to do when HF propagation
hits bottom ? I'll tell you how to continue enjoying the hobby despite the very poor propagation ...
Item three: A bandpass input filter, a well designed bandpass tunable input filter will improve many
receivers in a really dramatic way...Today's Dxers Unlimited's technical topics section will provide
you with information on a rather simple and easy to build tunable bandpass input filter that you can
add to any receiver... I have built several ones, and have already lost them to visitors to my
shack.. What has happened is that my visitors have wanted to borrow them to test with their radios
after seeing a hands on demonstration on how the tunable bandpass input filter works . And it was
easier to just give them away and build another one !!! Item four: ASK ARNIE, now la numero dos, and
almost la numero uno... Today's ASK ARNIE will answer a question sent in by a listener in Alabama,
USA... He wants to know if it was true or not that the old MFM hard drives are a treasure chest of
valuable electronic components... Stay tuned to listen to my answer ... Margarita Delgado is my
sound engineer and producer... I am Arnie Coro in Havana, back with you in just a few seconds...

.......

Si amigos, you are listening to the weekend edition of Dxers Unlimited , coming to you from
Havana... Solar cycle 23 is now going trough the downward slope towards solar minimum, and it is
producing lots of HF propagation disturbances, something that according to the propagation gurus is
quite normal... The fact is that for many weeks in a row HF propagation conditions have gone trough
the worst period of the whole cycle so far... The question asked by many of you is how to deal with
such poor propagation conditions... and here is what to do ...

in three easy and one not so easy steps ...

Step one: Start tuning your receiver from the highest possible frequency down, until you find the
first signals, and then mark that frequency.... During propagation disturbances , when the
ionospheric absorption index is high, you will notice that lower frequency signals are absent , and
that the stations you are able to pick up start around a certain cut-off frequency ... For example,
you may find that signals below 15 megaHertz are missing, and that your receiver just picks up
signals above 15 megaHertz... ... then you will almost for sure find that you can pick up signals up
to a certain frequency... say 22 or maybe 23 megaHertz... So, you have just found out about a very
important HF propagation concept.... useable bandwidth... In other words and to make it easier to
understand, the disturbance in progress when you tuned your radio has effectively wiped out signals
below 15 megaHertz due to the increase in free electron concentration of the D layer, while at the
same time, the number of free electrons found at the F2 layer is just high enough to support
propagation up to 22 or 23 megaHertz... Now, you realize that your best possible strategy is to find
the useable bandwidth at a given moment, and then look for stations that are located on frequencies
around 85 percent of the useable bandwidth if you are looking for the most stable reception, or go
all the way up to the edge of the MUF or maximum useable frequency if you want to really look for DX
!!! During the downward slope of a solar cycle, like we are going trough right now, the number of
solar events that cause HF propagation disturbances is higher than at any other phase of the
cycle... and that's why learning how to find the USEABLE BANDWIDTH of ionospheric propagation at any
given moment is so important when you really want to listen to the radio when conditions are not
good !!!

......

Welcome now to the Dxers Unlimited's technical topics section amigos !!! Today I will tell you about
a very simple, and rather easy to homebrew gadget... it is an external accessory that you can add to
any short wave radio.... and if you wish, you can also make another one to improve medium wave
broadcast band reception. This simple circuit consists of two or three inductance-capacitance tuned
circuits that are coupled using very small coupling capacitance ... Each of the tuned circuits is
independently tunable with an air spaced high quality variable capacitor... Adding such a tunable
bandpass input filter to any radio will improve reception on practical any receiver, but the
improvement is dramatical on the lower priced radios... That's why I have lost no less than five or
maybe six of the homebrew bandpass tunable short wave filters that I have built !!! The reason for
the loss is that when visitors come to my shack with their small portable radios and I connect the
radio to my fan dipole or my TTFD antennas inserting the tunable bandpass filter , people are so
amazed that they just want to take the filter back home and have it permanently installed... Because
building these filters is so easy ... I just give them away and simply build another one , as I only
need to find a few parts and then have the spare time to assemble a new one... Now listen carefully,
as I describe how these bandpass tunable input filters are made.

You will need two air spaced variable capacitors... The ones I use come from old radios, they have
two sections, one with a maximum capacitance of about 140 picofarads and the other section has a
maximum capacitance of 365 picofarads... You can find those twin air spaced variables in old radio
chassis, and they are fairly easy to remove. Brand new ones usually have two identical 365
picofarads sections and they are sold by several suppliers of parts for building crystal radio
sets... The coils for the tuned circuits are very easy to make, and you can use PVC pipe sections as
the coil forms. My favorite HF filter tunes from 5 to 18 megaHertz, and is very easy to assemble.
The signal from the antenna is fed to the first tuned circuit via a three turn link wound at the
bottom of the coil. I leave about 3 millimeters between the main winding and the link. The main
winding consists of 14 turns of number 18 enameled copper wire , closewound on a 25 millimeters or 1
inch diameter section of PVC pipe. The variable capacitor is connected to the coil so that the two
are in parallel... The first tuned circuit is coupled to the second tuned circuit using a small
ceramic capacitor of around 10 picofarads... actually I use a small ceramic trimmer capacitor that
has a minimum capacitance of 3 picofarads and a maximum capacitance of 15 picofarads, set so that if
will provide about 10 picofarads of coupling capacitance. The second tuned circuit is identical to
the first one, and the output to the radio is via a three turn link that connects to a small length
of RG58 U 50 ohm coaxial cable that is then connected to the receiver's antenna input... If you are
lucky to obtain the capacitors that have 140 picofarads and 365 picofarads , then you can add two
simple switches, so that the tuning range can be changed by using either the 140 picofarads or the
365 picofarads section.

For those of you that have never used a tunable bandpass input filter ahead of a portable low cost
transistor short wave receiver, here is my advise... You will find that after installing and using
the bandpass filter, you will never remove it again , because the improvement in reception that you
will notice is simply amazing.

Cross modulation problems , so characteristic of the small portables when you connect them to a good
outside antenna, will simply disappear amigos !!! And in many cases you will also notice that
reception becomes much less noisy, because the bandpass input filter will restrict the band of
frequencies that enters into your radio's input circuits... As a matter of fact, what you have done
when the bandpass tunable input filter was installed , is to include the five expensive and large
sized components that the radio's designer and manufacturer decided not to include for three good
reasons to them: ONE: Saving the cost of the big airspaced variable capacitors TWO: No need for two
additional tuning controls that will complicate the operation of the receiver and THREE: No need to
make a much bigger box for the radio in order to provide enough room for the two air wound coils and
airspaced variable capacitors !!!! NOW, here are YOUR THREE GOOD REASONS to homebrew your own
bandpass tunable input filter and place it ahead of your radio, between the external antenna and the
receiver. ONE: You will have TWO additional tuned circuits ahead of the radio's mixer TWO: You will
be able to peak the tuned circuits to the exact frequency that you are listening to . THREE: Adding
an external and easily removable box with the two tuned circuits is something really very easy to
do... SO, when you want to use the radio as a portable receiver, without the additional bandpass
tunable input filter, you can do it by just unplugging the filter from the radio's antenna input !!!

NOW, let me add two things: You can make a still better input filter using not two but three tuned
circuits, AND , you can add a built in low noise radiofrequency pre-amplifier stage after the filter
to compensate for the loss caused by the tunable filter. The low noise pre-amp is easy to build, and
you can include a variable gain control, but then the bandpass tunable input filter plus RF pre-amp
combination will require an external power supply !!!

I have here in my homebrewers files a small dot jpg file with the circuit diagram and instructions
on how to build a bandpass tunable input filter, one of the easiest to homebrew and more effective
accessories that you can add to any short wave radio !!!

.....

ASK ARNIE, yes ASK ME any radio hobby related question or questions you may have, send your
questions to arnie@rhc.cu, again arnie@rhc.cu, and here is now ASK ARNIE for today... YES , YES
amigo TODD from Alabama USA, the guy that told you to grab all those MFM hard drives of the early
years of personal computers was absolutely right !!! Those SEAGATE 10, 20,40 and 80 megabytes MFM
harddrives used by the old XT's and 286's are a real treasure chest of very high quality electronic
components, including very nice power field effect transistors... Just open up the hard drive
itself, and inside you will find a circuit board with large sized and easily removed parts that can
be desoldered with a 30 or 60 watt soldering iron... AND amigo TODD... then the only thing you will
have to do after removing the parts is to test them and keep them at hand for any of the upcoming
radio hobby construction projects !!!

And now amigos as always at the end of the show , here is Arnie Coro's Dxers Unlimited HF plus low
band VHF propagation update and forecast... Solar activity is now moving down, the K index will stay
above 4 for most of Sunday, and those of you at latitudes above 40 degrees North will continue to
suffer from rather poor HF propagation conditions. Sporadic E events are expected to continue at a
higher rate than last week, so look for TV and FM band DX this week !!! 6 meter and 10 meter band
operators should see a lot of E skip too... See you all at the mid week edition of the program
amigos and don't forget to send your signal reports and comments about the show to arnie@rhc.cu or
VIA AIR MAIL, a postcard will be fine to Arnie Coro, Radio Havana Cuba, Havana, Cuba.