[Skimmertalk] Red Pitaya 1:14 impedance transformator + SV1AFN pre-amplifier

Tue Oct 24 01:17:53 EDT 2017

Lee,

Thanks for the detailed report!

It sounds like you're locating the FM stubs at the preamp output ("between
the pre-amp and the splitter") instead of at the preamp *input*, where they
may be more effective.  The goal is to keep FM broadcast signals out of the
pre-amp, where they are likely to be mixed into unwanted HF spurious
signals.

Assuming a 50 ohm antenna, a 1/4 wave 50 ohm stub made of RG-8 or better
could provide a deeper null than RG-6.  Better yet, try a 50 ohm lowpass
filter, the type we used to use to reduce TVI from our transmitters during
the old days of VHF analog TV (with a 54 MHz cutoff), on the pre-amp
input.  The lowpass may work better than the stubs, as placement at the
right point in the line would be less critical then it is with stubs.

73,
Bob, N6TV

On Oct 23, 2017 9:53 PM, "Lee Sawkins" <ve7cc at shaw.ca> wrote:

Evaluation of Red Pitayas with 20 dB pre-amp, 14:1 matching transformers
and stub filters for removal FM band interference.  The Red Pitaya's input
attenuators were bypassed. (2 - 5)Today I did some testing on my Red Pitaya
system using the test method I proposed.  I have a pair of Red Pitayas
connected to a multi-band vertical antenna via a 75 ohm CATV splitter.
Although there is a slight mismatch, the price was right.  I tried various
combinations, with and without 14:1 matching transformers, 20 gain pre-amp
and stub filters.  The stub filters were 1/4 wave at 95 MHz.  RG6 and
open.  I get a lot of interference from FM stations which are aliasing into
the desired frequency range.  The interference is especially bad on 10
meters but also shows up on the lower bands as well.  The pre-amp was
placed between the antenna and the splitter.  The stubs were placed between
pre-amp and the splitter.
160 and 80 are not mentioned in the following testing.  They are still
being worked on.  I believe there is a small amount of intermod on these
bands, not related to the FM band signals.The signal source for S/N was
VE7MTY/B on 28.197 MHz.  This beacon is 7.8 miles (12.6 km) from me.  This
signal is S7 on the HDSDR with no pre-amp in line.  The band noise does not
fluctuate.Test #1 No pre-amp, no matching transformers, no stubsS/N 36 dB
no spurious signalsTest #2 No pre-amp, 14:1 matching transformers, no
stubs.S/N 36 dB, no spurious signalsThis was surprising.  The HDSDR
confirms that the signal level is exactly the same to the SDR, with or
without matching transformers.Test #3 20 dB pre-amp, no matching
transformers, no stubsS/N 49 dB,  Spurious signals on all bands.  On 10
meters spurs from 28030 - 28190, 28125-28300.  Peak spur about 28193, 20 dB
above band noise floor.  Other bands not as bad.
Test #4 20 dB pre-amp, no matching transformers, 1/4 wave open stub for 95
MHz connected to T connector between pre-amp and splitter.S/N 48, spurious
signals dropped about 13 dB, still 7 dB above band noise at worst
frequency  Signal dropped by 1 dB in level.Test #5 20 dB pre-amp, no
matching transformers, two 1/4 wave open stubs for 95 MHz connected to two
T connectors located 5.5 ft apart.  This 5.5 ft is slightly shorter than a
quarter wave at 28 MHz.S/N 49 dB,  worst spurious signal about 1 dB above
noise floor, got the 1 dB loss back when was using only one stub.Test #6 20
dB pre-amp, 14:1 matching transformers, no stubs.S/N 54 dB, spurious
signals everywhere like the no transformer case, but about 10 dB lower in
level.  Signal at 28.193 is now only 10 dB above band noise floor. Test #7
20 dB pre-amp, 14:1 transformers, two 1/4 stubs as before.S/N 54 dB, no
detectable spurious 40 meters and up.Conclusion.Using a 20 dB pre-amp, 14:1
transformers and two 1/4 stubs for FM b
 and can improve the S/N by 18 dB on 10 meters without any spurious
signals.  I doubt that a 10 dB pre-amp, as some suggest, has enough gain.
I did not test this.
Lee VE7CC
----- Original Message -----
From: DD5XX (Saki) <dd5xx at web.de>
To: skimmertalk at contesting.com
Sent: Mon, 23 Oct 2017 03:59:53 -0600 (MDT)
Subject: Re: [Skimmertalk] Red Pitaya 1:14 impedance transformator + SV1AFN
pre-amplifier

Hi Lee,

the key point is "reliable" signal :) I have also some nearby local RBN
skimmer but after long-ranged research I found that their local QRM
level differs heavily. Some of them have really big gaps between
day/time (up to 10dB!), others up to 3-4dB. There is only one single RBN
I am aware of that replies with constant signal levels during a 24h
period. I can use such a node for "realiable" tests. The same applies to
some WSPR beacons out there, I did also detailled tests on that because
I was curious how high the local noise on some beacons is. On my own
testing station the local noise is noticeable, too. That's because I
live in the city with lots of man-made-noise. In the late hours where
man-made-noise are low, the SNR reports are much better of course. The
OM should always have in mind that the reports are "Signal-TO-NOISE"
ratio. That's exactly the reason why currently I am planning in
installing a CW-RBN/WSPR receiving node on a very good low-noise area
onto a tower, hopefully to get constantly SNR values.

Thanks for sharing your thoughts Lee. Have a nice week and best 73

Saki, DD5XX

Am 23.10.2017 um 10:02 schrieb Lee Sawkins:
> I have found an easy way to accurately measure exact s/n ratio changes in
any skimmer system. This only requirement is a having a beacon or other
reliable signal within ground wave range. I have a beacon on 10 meters that
I use. Simply see what the skimmer is giving for a s/n report for this
signal. Then make your changes such as adding a 14:1 transformer, a
pre-amp, or an inline filter etc. After each change, wait a while and see
what the new s/n report is. The difference between the new and old reports
is the exact difference caused by whatever you did. This method is
accurate, repeatable, and requires no test equipment.
>
> Lee VE7CC
>
> ----- Original Message -----
> From: DD5XX (Saki) <dd5xx at web.de>
> To: skimmertalk at contesting.com
> Sent: Mon, 23 Oct 2017 01:15:09 -0600 (MDT)
> Subject: Re: [Skimmertalk] Red Pitaya 1:14 impedance transformator +
SV1AFN pre-amplifier
>
> Hi Martin,
>
> thanks for feedback. I think I will order one from SV1AFN. I already did
> send him an email with some questions yesterday. Hopefully will order
> one this week. Thanks again and have a nice week.
>
> Best 73
> Saki, DD5XX
>
>
> Am 23.10.2017 um 01:32 schrieb Martin Rath:
>> Hi Saki,
>>
>> I use Maki's J310 LNA in front of the red pitaya for 2 2 reasons:
>> 1) additional gain. As per Maki himself the res pitaya requires an
>> additional 5-10 dB amplification.
>> 2) to protect the inputs: less than 2 were after i got the RP a close
>> by lightning took out the AD8066 op amp. I did manage to replace don't
>> want to so that all too often: Singapore gets hit by major t-storms
>> over 200 days per year. Running a skimmer here is not always easy as i
>> cannot always be home to unplug the antennas. I would much rather have
>> to replace a J310 than the AD8066 another time.
>>
>> I'm happy with the performance. I'm still waiting for one 14:1
>> transformer but it's already on the way.
>>
>> 73's Martin 9V1RM
>> On Mon, 23 Oct 2017 at 00:13, DD5XX (Saki) <dd5xx at web.de
>> <mailto:dd5xx at web.de>> wrote:
>>
>> Hello all,
>>
>> thanks to Bob (N6TV) I got one of those Red Pitaya impedance
>> transformator 1:14. I wonder if anyone did try to add a
>> pre-amplifier on
>> the input and if so what is your experience ? I have heard and did
>> read
>> lots of positive feedback on the pre-amplifier offered by SV1AFN
>> on his
>> website. I would be interested in the kit "J310 push-pull HF pre-amp
>> 0.5-54MHz" (see "https://www.sv1afn.com/j310preamp.html"). Do you
>> think
>> to gain some more dB performance with this combination ?
>>
>> Red Pitaya INPUT1 <---- 1:14 impedance transformator <---- SV1AFN
>> pre-amp
>>
>> Looking forward into your comments.
>>
>> 73
>> Saki, DD5XX
>>
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--
Vy 73s de DD5XX
Saki

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