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Author Topic: Selectivity & Sensitivity - in used low cost HF radio  (Read 1637 times)
N1KCG
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« on: March 07, 2013, 05:22:56 AM »

Does any Elmer have a listing of used LOW COST (below $500) HF radios that have a very good sensitivity and selectivity? Would like to find discussions or articles, or advice blogs about this.  Am sure someone must have raised the issue of developing some kind of reference list before.  Completely configured radio references that include all options would be best to reference, as know now that many radios require three or four filters before their parameters improve.

Also would like to know what are the significant change levels of sensitivity or selectivity that one should be concerned with.  From my old electronics class can remember that the 3dB down point is when the logarithmic change is significant, so a radio with a difference in this statistic should be greater than this, to make any statistical difference.

If I am not thinking about this correctly, please make that comment.  Anything positive and constructive on understanding this is appreciated.
   
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N4NYY
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« Reply #1 on: March 07, 2013, 05:26:39 AM »

I would probably use something like this as a guide:

http://www.sherweng.com/table.html

That have a lot of non-production radios that may be in the price range.
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K8GU
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« Reply #2 on: March 07, 2013, 05:41:37 AM »

This is an academic discussion at this point.  What are you trying to do on the air?  But, without any further guidance, old "contest-grade" radios with appropriate filters are probably your best bets, some examples:  TS-830S, TS-930S, IC-765 (might be a bit high in price), FT-990 (also probably a bit high).  Most of these rigs will also take an aftermarket Inrad roofing filter, something that will definitely improve performance (speaking from experience with my TS-930S).

The Sherwood list is great, but it's not perfect.  He has some caveats on the site (about the small sample size, etc).  But, the bigger thing is that his tests only approximate a single real-world situation (trying to copy a CW signal at the noise floor N kHz away from a very loud signal).  That may not be at all relevant to the operating you wish to do.
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KE3WD
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« Reply #3 on: March 07, 2013, 06:05:45 AM »

Lots can be done through proper usage of Passband Shift. 

Also, IF DSP. 

The good old Alinco DX-70 series of rigs can be found for less than the quoted price and has rather good Selectivity and Sensitivity. 

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K5LXP
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« Reply #4 on: March 07, 2013, 06:46:51 AM »

LOW COST (below $500) HF radios that have a very good sensitivity and selectivity?

You defined "low cost" but not "very good".  So you will end up with what everyone else thinks is "very good", whatever that is.


Mark K5LXP
Albuquerque, NM
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WA3SKN
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« Reply #5 on: March 07, 2013, 07:46:50 AM »

Nowadays the HF sensitivity issue is controlled by atmospheric or other external sources, so any of the radios will be OK.
And selectivity needed is controlled by mode. All have a design cw passband of 500 Hz so it will do RTTY. For SSB you will want about 3 kHz, while AM will want 6-10 kHz.  And FM will be wanting 15 kHz.  This is best (and most expensively) done at the first IF with 8+ poles of filters.  However, the challenge is how to do it cheaper.  If you make a single mode radio you can get the selectivity you want, but if you want to have it multimode, then there will be compromises or cost.  (Cheaper, faster, better... pick any two!)
It is amazing what they can do in todays radios... but you might want to pick up a used rig and redesign for a single mode.
73s.

-Mike.
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WB2WIK
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« Reply #6 on: March 08, 2013, 01:55:00 PM »

They all have plenty (enough) sensitivity once connected to a real outdoor antenna.  The difference between the "most sensitive" and the "least sensitive" fairly modern HF transceivers is too small to notice.

Selectivity is a completely different issue. Wink

If you don't mind "tuning up" a transmitter, some of the best-performing HF transceivers in that price range are "hybrids" which are all solid state except the final parts of the transmitters.  Kenwood TS-830S, Yaesu FT-101ZD or FT-901DM, Yaesu FT-102 all spring to mind as rather spectacular rigs in almost all ways, but they do have a few tubes and will require transmitter tuning.

Also in that range may be the Icom IC-740, which is all solid state and an excellent rig in most all ways.  Ditto the Ten Tec Triton-IV or OMNI-D.
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G3RZP
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« Reply #7 on: March 08, 2013, 02:38:37 PM »

The FT102 is old. It is a very good receiver, but has problems with the relays. Some people change the relays, even though something like 8 components ( a small RF choke, a capacitor and some resistors) on the bottom of the PCB are cheaper and easier to install. It has a major problem with a spurious response on 3.5MHz CW in that strong signal at 3500 will still breakthrough as you tune up the band to about 3505. That can be fixed, but it needs technical knowledge and capability. The other big CW problem (besides totally antisocial key clicks, which can be fixed) is that the CW xtal osc can be slow to start as the crystal ages: this needs a resistor change in the source of the FET in the oscillator.

If you aren't an 80m CW DXer, these problems won't worry you.
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