[hpsdr] Preamplifier Schematic source

[Phil Harman]

Hi Steve,

Thanks for your most interesting comments on active antennas and in particular the use of passive ones.

Whilst fitting a HPF, to reduce MW broadcast signals, between the whip antenna and amplifier is not practical (due to the high impedance levels) fitting one between the output of the amplifier and an HPSDR is.  Whilst this is too late to remove any IMD effects in the amplifier it does help in preventing overload of the HPSDR ADC.

73's Phil....VK6APH 
  ----- Original Message ----- 
  From: Steve Hubbard 
  To: hpsdr at lists.openhpsdr.org 
  Sent: Wednesday, March 24, 2010 7:49 PM
  Subject: Re: [hpsdr] Preamplifier Schematic source


  ***** High Performance Software Defined Radio Discussion List *****




------------------------------------------------------------------------------


  Regarding receiving antennas for HPSDR, I think it would be hard to find an active antenna to match the linearity of the HPSDR front end. Another issue with active antennas that have a high input impedance is that they tend to have a constant effective height (output voltage divided by field strength) down to very low frequencies. This will bring in local MF broadcast stations at a very high level, which could overload the ADC. 

  In the HF band, atmospheric noise dominates at quiet locations, particularly at low frequencies and the level of noise rises more rapidly below 1 MHz. For this reason it is acceptable for a receive antenna to be severely mismatched. In fact it is desirable to have the gain fall away at lower frequencies because you only need enough effective height to bring the external noise above the noise floor of the receiver. 

  Seeing the active antenna using a Norton amplifier at http://www.kongsfjord.no/dl/dl.htm reminded me of some work I did with passive receive antennas using whips about 5 m high. I recall that a vertical element of this height combined with a 9:1 or 16:1 impedance (3:1 or 4:1 turns) transformer makes quite a good broadband receive antenna, even without an amplifier. The high termination resistance reduces the mismatch loss away from resonance and flattens the resonant peak. With no active components the linearity can be superb. I would use a ferrite core with a moderate permeability as I've found that nickel-zinc ferrites such as Ferroxcube 4C65 introduce intermodulation due to their hysteresis curve. Not sure I'd use bell wire as a transmission line though.

  For anyone wanting to do calculations, a whip that is much shorter than a quarter wave will have an emf equal to the field strength times half the height. The source impedance will be around 10 pF/m in series with a few tens of ohms. Any mast that elevates the base of the whip will greatly increase its effective height.

  73 de Steve, VK5SH (ex VK6ABZ)






------------------------------------------------------------------------------


  _______________________________________________
  HPSDR Discussion List
  To post msg: hpsdr at openhpsdr.org
  Subscription help: http://lists.openhpsdr.org/listinfo.cgi/hpsdr-openhpsdr.org
  HPSDR web page: http://openhpsdr.org
  Archives: http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/


------------------------------------------------------------------------------



  No virus found in this incoming message.
  Checked by AVG - www.avg.com 
  Version: 9.0.791 / Virus Database: 271.1.1/2760 - Release Date: 03/21/10 03:33:00
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openhpsdr.org/pipermail/hpsdr-openhpsdr.org/attachments/20100327/d6909f64/attachment-0003.htm>