EMRFD Message Archive 1468
Message Date From Subject 1468 2008-02-23 20:55:18 Art low loss high Q front end for vhf
This is my first post here, I hope it is not to far off topic.
I read THE BOOK, and I have to say it enlightened me, in many ways.
Thanks for taking the time to put it together!
At this time, I am interested in building a high performance (mostly
sensitive) receiver for the 128 to 138 MHz range, for the purpose of
listening for possible sporadic E skip that might occur on the nearby
2 meter band.
I am avoiding the usual (more common) monitoring of the FM broadcast
band for this purpose because propagation on the FM broadcast band
says very little about any propagation that might occur on the 2
meters, it's just to far removed from 144 MHz. For the reason, I am
not interested in monitoring anything under 128 MHz. A side benefit
of monitoring higher frequencies (well removed from the FM broadcast
band) is that I do not have to worry about the megawatt transmitters
that are there.
I plan to build an aircraft band converter and use it with a softrock
receiver in the 5 to 10 MHz range.
I read the Wes publication about building high performance vhf
filters, but these have to much insertion loss and will largely
negate the usefulness of the 1 db noise figure converter I hope to build.
Is there anyway to use the same technology to obtain a lower
insertion loss at the expense of some of the narrowband filter response???
The upper end of the aircraft band has a few strong signals in it,
but they tend to be intermittent rather than constant. My feeling is
that I don't need a high level mixer, and was thinking of using an
SA602 because it has built in AGC.
If I had some copper pipe (say 2 inch ID) silver plated on the inside
and used stranded center conductor from silver plated RG8 sized coax
to make the inductors, would my Q be reasonable???
Basically, I need to know if there is anyway to obtain a lower
insertion loss front end filter for a wider bandwidth front end?
My hope is to have the softrock be a variable IF, so that my DDS can
be the LO and I avoid higher frequency LO's altogether. I am working
towards building a Brainerd DDS, since they are now available as
kits-and I know the guy who manages the group buys for that DDS! The
AD9951 should make a dandy LO for a softrock!
Appreciate any comments.
Thanks,
Art.1469 2008-02-25 07:11:21 Allison Parent Re: low loss high Q front end for vhf 1471 2008-02-26 12:40:43 Art Re: low loss high Q front end for vhf
> > This is my first post here, I hope it is not to far off topic.
> >
> > I read THE BOOK, and I have to say it enlightened me, in many ways.
> > Thanks for taking the time to put it together!
> >
> > At this time, I am interested in building a high performance (mostly
> > sensitive) receiver for the 128 to 138 MHz range, for the purpose of
> > listening for possible sporadic E skip that might occur on the nearby
> > 2 meter band.
> >
> > I am avoiding the usual (more common) monitoring of the FM broadcast
> > band for this purpose because propagation on the FM broadcast band
> > says very little about any propagation that might occur on the 2
> > meters, it's just to far removed from 144 MHz. For the reason, I am
> > not interested in monitoring anything under 128 MHz. A side benefit
> > of monitoring higher frequencies (well removed from the FM broadcast
> > band) is that I do not have to worry about the megawatt transmitters
> > that are there.
> >
> > I plan to build an aircraft band converter and use it with a softrock
> > receiver in the 5 to 10 MHz range.
> >
> > I read the Wes publication about building high performance vhf
> > filters, but these have to much insertion loss and will largely
> > negate the usefulness of the 1 db noise figure converter I hope to
>build.
> >
> > Is there anyway to use the same technology to obtain a lower
> > insertion loss at the expense of some of the narrowband filter
>response???
>
>Yes you put it after the LNA for image stripping. Before the LNA use
>enough (and no more) selectivity to keep out the bad guys. Problem
>is often wide tuning range and good front end selectivity are interactive.
>
> > The upper end of the aircraft band has a few strong signals in it,
> > but they tend to be intermittent rather than constant. My feeling is
> > that I don't need a high level mixer, and was thinking of using an
> > SA602 because it has built in AGC.
>
>No it does not have AGC. It can be good for that but the overload
>level will be very low. I would use a stronger mixer as at VHF
>there are a lot of signals and even the aircraft side (between
>137 and 144 is military) has a lot of issues with strong signals.
>Also I'd recommend a higher IF at least 14 to 20Mhz or more as
>images are a problem. If memory serves Softrock is 40M and 7mhz
>is too low for really good image response at 140mhz.
I just got done compiling a list of all the known signals I might
hear on 128 to 138 MHz-which is every airport East of the Mississippi
River. My plan is to use a horizontal polarized antenna, so any
signals that aren't E-skip will be attenuated by the polarization
difference. I also have plans for a 2 or 3 element LPDA, which will
be designed for the same frequency range and will be directional with
modest gain.
In my list, I find no aircraft band signals between 136 and 138 MHz,
I believe that segment is used for wefax satellite downlinks.
So, it's possible I will build my receiver for 125 through 135.99 MHz.
It's also possible I will end up with 2 receivers with a common
antenna and preamp, especially if the system is automated and
operates from the same computer.
I have a nice diagram of a 2 meter converter (144 Mhz to 28 Mhz). It
uses a dual gate mosfet. Would it be that much better than an SA602??
Here is the converter I actually prefer, but I don't know if it is
any better than the SA602. If you could give it the once over, I'd
appreciate your comments.
http://homepage.eircom.net/%7Eei9gq/vhf_conv.html
> > If I had some copper pipe (say 2 inch ID) silver plated on the insideMy hope is that I will use the DDS to be a LO for the softrocks, so
> > and used stranded center conductor from silver plated RG8 sized coax
> > to make the inductors, would my Q be reasonable???
>
>Use solid conductor Qs in the 1000-2000 range are attainable without
>plating. Plating is mostly for preventing corrosion. Not with Qs
>that high narrow tuning range is the expected result.
>
> > Basically, I need to know if there is anyway to obtain a lower
> > insertion loss front end filter for a wider bandwidth front end?
> >
> > My hope is to have the softrock be a variable IF, so that my DDS can
> > be the LO and I avoid higher frequency LO's altogether. I am working
> > towards building a Brainerd DDS, since they are now available as
> > kits-and I know the guy who manages the group buys for that DDS! The
> > AD9951 should make a dandy LO for a softrock!
>
>Make ssure the DDS has a low phase noise source as that can degrate
>that 1db NF also.
the dds will only need to be 4 times the desired LO frequency.
Everything will be converted down to a frequency the softrock can
handle and the softrock will be tuned by the DDS. The final IF will
be a variable IF.
Regards,
Art1472 2008-02-26 15:00:58 Graham Re: low loss high Q front end for vhf
I think the problem you will run into is a systems issue, in that
all of those VHF base stations you want to monitor are talking
to airplanes at up to 45,000 feet in the air that reply on those
same frequencies. From 45,000 feet, radio line of sight is in the
400 mile+ range, no skip needed. So you are going to hear a
lot of strong signals on those frequencies, and unless you can
figure out a way to tell an airplane signal from a base signal,
you won't get what you are looking for out of the system.
1480 2008-02-29 06:52:28 kerrypwr Re: low loss high Q front end for vhf
If you want to go the "mechanical" route with a helical resonator,
this is a good article;
http://www.qsl.net/yu1aw/low_noise.htm#XHD
which also gives a good overview of "Q" etc.
I intend to build something similar using 45 mm copper tube as the
housing; I have done some of the machining on it but the project is
"on the back burner" at present.
A suitably-retuned two-metre pre-amp would do well, eg;
http://www.minikits.com.au/kits2.html#eme157
You could also look at one of the Wefax receivers that are available, eg;
http://www.minikits.com.au/kits3.html#eme117
MiniKits is, of course, in VK but I'm sure that Mark VK5EME would be
delighted to do some overseas business; his prices are reasonable and
he charges minimal postage.1483 2008-02-29 06:52:51 Allison Parent Re: low loss high Q front end for vhf
40W and the guy at 45000 feet is not the problem but the little puddle
jumper at 2500ft is close and always a problem even at low power.
Fyi: I used a superregen for listening to aircraft band for years and
with a 23" whip inside the house the range for hearing aircraft was
typically 50mi or more.
Personally tuning a cavity over that range is more of a challenge
as varicap diodes have relatively low Q compared to air variable
caps and would tend to significantly lower the Q of the cavity
and also can add their own IM products if there is enough RF
around.
Myself I monitor the beac1484 2008-02-29 08:57:14 Art Re: low loss high Q front end for vhf
I'm aware of the non-linear aspect of the varicap. However, it can't
be any worse than a gasfet, and there are tons of them in use. It's
an experiment, and the loss in Q or the degraded IM products might
turn out to be an issue. I plan on a resonant antenna, so there will
be a certain amount of selectivity built into antenna.
It might be more practical to build a tunable helical resonator, and
tune the cap with a small motor-that give me lots of Q but adds a
mechanical factor.
The attractiveness in the aircraft band as a propagation monitor is
that the ID's are frequent and any aircraft with a transponder aboard
can be tracked in real time via the internet. Although it might be
possible to have a fully automated system scan the 2 meter beacon
band, I am not aware of anyone using such a system. But, it's
probably doable with todays technology. There aren't that many
beacons on 2 meters tho, so some openings could be missed-even with automation.
Again, thanks. All comments are appreciated.
Regards,
Art
>Actually most of those stations are typically in the range of 10 to
>40W and the guy at 45000 feet is not the problem but the little puddle
>jumper at 2500ft is close and always a problem even at low power.
>
>Fyi: I used a superregen for listening to aircraft band for years and
>with a 23" whip inside the house the range for hearing aircraft was
>typically 50mi or more.
>
>Personally tuning a cavity over that range is more of a challenge
>as varicap diodes have relatively low Q compared to air variable
>caps and would tend to significantly lower the Q of the cavity
>and also can add their own IM products if there is enough RF
>around.
>
>Myself I monitor the beac1485 2008-02-29 16:13:08 Art Re: low loss high Q front end for vhf
The EME117 looks nice, although it is currently out of stock. And, I
don't know how expensive it is.......
I just ran across a Norton amplifier link that takes a high input
level, uses a BJT and is very low noise preamp. It uses a
transformer, and looks very clever although it is primarily used at
HF. W1JR wrote about it years ago-and he only does vhf.
Traditionally, a toroidal transformer is used, but I am wondering if
a standard air dielectric transformer made out of large gauge silver
plated copper wire can be used instead of the toroid.
I actually have a spare ARR gasfet preamp that might be used if it
was retuned. I would have to build a helical resonator for it, and
send the pair of them to ARR for retuning I suspect.
Again, thanks and thanks to all on the list-the topics here are very
interesting and I've learned allot by just reading the mail.
Regards,
Art1487 2008-03-01 10:02:14 w4zcb77 Re: low loss high Q front end for vhf
With his DXCC totals, I'm pretty sure he would object to that
characterization.
> I actually have a spare ARR gasfet preamp that might be used if itYour helical resonator is in the mail. Take a look, you may have to cut
> was retuned. I would have to build a helical resonator for it, and
> send the pair of them to ARR for retuning I suspect.> Regards,
>
> Art
>
a turn off the helix for your frequency. I made them for VCO's at 70-
100 MHz. Built some for 2 meters, the helix is 7 turns for those.
Regards
W4ZCB1491 2008-03-01 22:25:45 Art Re: low loss high Q front end for vhf
tend to regard him as a vhf expert rather than an HF one. However,
the error is mine.
I look forward to receiving the helical resonator.
I found the YU1AW's qsl.net articles, it is a real treasure. I'm
thinking about using your resonator and his BF998 dual gate mosfet
front end for my receiver. He also wrote some other nice articles
that are on the website too, including one that discusses the Q of
the input circuit and what happens to that Q when it is connected to
the transistor and how the ratio of the loaded Q to the unloaded Q
affects the input tuned circuit losses. So, I'm making good progress.
It looks like the first stage will be your resonator used with
YU1AW's BF998 dual gate mosfet vhf preamp (especially attractive
because I have a bunch of them in my junkbox already). The mixer can
be nearly the same circuit, with the LO fed into one of the gates and
the output tuning adjusted to favor the down converted frequency
which will be somewhere in the 15 to 30 MHz range.
Not sure whether I can use the BF998 for the third (and final IF) or
whether other devices are more suitable for the lower frequencies.
But, I'm miles ahead of where I was just a short week ago! I have
finally gotten over the temptation to try to use an SA602 type mixer
too-I'll let others use them instead and stick with discrete components::>
Thanks to you and the entire list for some nudges in the right direction!
Regards,
Art1496 2008-03-09 15:57:17 Art Re: low loss high Q front end for vhf
It is an awesome looking job!
I wonder how a 4 inch copper pipe would do with simple 1/4 inch or
3/16 inch tubing wondered to make the coil??
Also, would the ultra low resistance mosfets (with milliohm on
resistance) be good for switching a capacitor in to tune the
resonator for the different ranges I need? I don't think they conduct
until the applied voltage exceeds .7 volts. Or would a varactor give
a higher Q, I did manage to find some low pF range varactors that
have Q's over 250 when tuned at 30 v reverse bias.
Thank again for the helical resonator and for all those that made
comments on the list.
Art
>Your helical resonator is in the mail. Take a look, you may have to cut[Non-text portions of this message have been removed]
>a turn off the helix for your frequency. I made them for VCO's at 70-
>100 MHz. Built some for 2 meters, the helix is 7 turns for those.
>
>Regards
>W4ZCB
>
>
>No virus found in this incoming message.
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>2/27/2008 8:35 AM
1499 2008-03-10 08:12:23 Kenneth Stringham Re: low loss high Q front end for vhf
voltage switchings could be high enough to destroy
them. Most tuners uses relays rated for the types of
voltages one would expect in this application.
Ken - AE1X
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