HF pre-selector

Useful information regarding antennas for SDR products.
CaptainNemo
Posts: 114
Joined: Mon Jan 05, 2015 1:22 pm
Location: Genoa - Italy

HF pre-selector

Postby CaptainNemo » Tue Jun 23, 2015 12:23 pm

Someone knows / has experience of a good HF pre-selector for SDRplay?
It would be nice if it works also for LW/MW.

bye
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CaptainNemo
Posts: 114
Joined: Mon Jan 05, 2015 1:22 pm
Location: Genoa - Italy

Re: HF pre-selector

Postby CaptainNemo » Wed Aug 26, 2015 1:46 pm

under "new users' experience" topic Eckhard (user eckrot) wrote:

"Dear SDRPlay team: For the next revision, please spend a few cent more and give it better filters. Thanks!"

So, I'm back again with my question:
-- can an external pre-filter improve greatly the performances of sdrplay? (I suppose yes...)
-- anyone has direct experience of this issue? if yes, can you share with the community a suggestions for a good commercial pre-filter to buy online?
-- has sdrplay team plan to improve the "on board" filtering in a future version of RSP?
-- maybe sdrplay team can open a new line of business developing an external pre-filter (manually tunable of course)...
(this will save the "old" owner investment...)
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jon
Posts: 293
Joined: Tue Jan 06, 2015 10:48 am

Re: HF pre-selector

Postby jon » Wed Aug 26, 2015 9:35 pm

Hi captain Nemo. Many thanks for the suggestions. Have you thought about a simple ATU? You can pick them up cheaply on eBay and they can provide a lot of extra selectivity and hence filtering. Best regards, Jon
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CaptainNemo
Posts: 114
Joined: Mon Jan 05, 2015 1:22 pm
Location: Genoa - Italy

Re: HF pre-selector

Postby CaptainNemo » Thu Aug 27, 2015 2:18 pm

Thank you for your answer.
I suppose ATU means Antenna TUner (e.g. MFJ-956).
It is the kind of thing I'm looking for.
Maybe someone in the community could have some direct experience ...

Bye
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Tech_Support
Posts: 202
Joined: Mon Jun 01, 2015 7:00 pm

Re: HF pre-selector

Postby Tech_Support » Thu Aug 27, 2015 6:23 pm

CaptainNemo wrote:under "new users' experience" topic Eckhard (user eckrot) wrote:

"Dear SDRPlay team: For the next revision, please spend a few cent more and give it better filters. Thanks!"

So, I'm back again with my question:
-- can an external pre-filter improve greatly the performances of sdrplay? (I suppose yes...)
-- anyone has direct experience of this issue? if yes, can you share with the community a suggestions for a good commercial pre-filter to buy online?
-- has sdrplay team plan to improve the "on board" filtering in a future version of RSP?
-- maybe sdrplay team can open a new line of business developing an external pre-filter (manually tunable of course)...
(this will save the "old" owner investment...)


Dear CaptainNemo,
Improved RF pre-selection will help reduce the possibility of spurious responses when operating in the upper reaches of HF band (30 MHz - 60 MHz) and in the range of 250 MHz - 430 MHz. This is particularly true if the pre-selector is effective at suppressing VHF FM signals, which are the main cause of spurious responses. The impact in other bands is likely to be minimal and it is important that any external filter has minimal insertion loss to ensure the lowest possible impact on receiver sensitivity.
We greatly appreciate all suggestions for improving the product and we will look to incorporate as many of these suggestions as possible. We do intend to develop new versions of the RSP, but at this stage, we cannot give a schedule.

Sincerely

SDRplay Tech_Support
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CaptainNemo
Posts: 114
Joined: Mon Jan 05, 2015 1:22 pm
Location: Genoa - Italy

Re: HF pre-selector

Postby CaptainNemo » Fri Aug 28, 2015 8:48 am

TNX very much to tech support!

Since my interest is mainly in HF (3-30 MHz)
from your kind answer
I understand
pre-filtering in HF is useful therefore NOT a big advantage.

bye
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Tech_Support
Posts: 202
Joined: Mon Jun 01, 2015 7:00 pm

Re: HF pre-selector

Postby Tech_Support » Fri Aug 28, 2015 1:20 pm

Dear Captain Nemo,
The pre-selection that segments the lower end of the HF spectrum into 0-12 MHz and 12-30 MHz, will certainly help prevent the front end from overloading. Adding more pre-selection filters for the more of the individual amateur bands is of questionable value though as one key feature of the RSP is its ability to handle up to 8 MHz of spectrum. So for example, if we were to add SAW filters to provide RF separation of the 30m, 40m, 60m and 80m bands, you would gain in ultimate selectivity, but you would also lose the ability to monitor 8 MHz of spectrum as the pre-selection filters would limit you to a few hundred KHz in each band. So, it is really a question of trying to find a reasonable compromise.

Sincerely

SDRplay Tech_Support
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CaptainNemo
Posts: 114
Joined: Mon Jan 05, 2015 1:22 pm
Location: Genoa - Italy

Re: HF pre-selector

Postby CaptainNemo » Fri Aug 28, 2015 6:47 pm

I hope to find soon a cheap/used external pre-selector (I mean no more than 100 euros more or less)
then I will try to report if it is useful or not
for swling.
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n3ikq
Posts: 1
Joined: Fri Sep 04, 2015 12:55 am

Re: HF pre-selector

Postby n3ikq » Wed Sep 16, 2015 2:42 pm

First, do you have a reason to need a pre-selector? Are you plagued with specific noise? Are you a "roll your own" kind of guy or do you want to skip the hands on part and purchase? Do you want a pre-selector or would a directional loop antenna be better? First let me assure you that "jumping in" to filter building (and loop antenna building) is a very rewarding and fun process! I spent decades avoiding filters because of their mathematical complexity but with all of the online information and calculators it's easy to just copy-cat other designs. The learning kind of happens naturally as you build your filters. I built a low pass filter that blocks everything above 600 kHz so I can hunt beacons without being bothered by strong BCB stations. It works very well with the SDRPlay. Online you will find tons of information about how to build low pass, band pass and high pass filters as well as how to wind torroids. At these relatively low frequencies, construction is pretty easy even on perfboards. It does help a lot to get hold of a capacitance and inductance meter to help you measure the exact value of the hand wound inductors and capacitors. With a meter you can fine tune your coils and find the best capacitors in your stash but even using parts bin parts you can get a filter to work pretty well at lower frequencies. The exception is that you need to buy the right torroids for the job as they are frequency dependent. Ebay has all of the torroids and caps you would ever need to build your own as well. Here are some great sources of information I have in my Favorites list:
http://www.k5td.com/K5TD/500kHz_Low_Pass_Filter.html
http://www.ok1rr.com/index.php/technica ... top-filter
http://www.robkalmeijer.nl/techniek/ele ... index.html
http://www.frontiernet.net/~jadale/Loop.htm
http://www.qsl.net/kp4md/lpfilter.htm
http://members.shaw.ca/ve7sl/burhans.html
http://www.cliftonlaboratories.com/z100 ... filter.htm
http://www.pssurvival.com/ps/radio/ante ... s_2004.pdf
http://www.dxing.com/tnotes/tnote06.pdf
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CaptainNemo
Posts: 114
Joined: Mon Jan 05, 2015 1:22 pm
Location: Genoa - Italy

Re: HF pre-selector

Postby CaptainNemo » Mon Oct 05, 2015 6:32 am

Hi N3IKQ many thanks for you kind answer.
I was looking for "direct" experience with "sdrplay+prefilter".
I understand you have direct experience with low pass filter with good result for your interest in NDB.
Thanks for sharing this.
Bye
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