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<DIV><FONT size=2 face=Arial>Hi Steve,</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>Thanks for your most interesting comments on active
antennas and in particular the use of passive ones.</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>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.</FONT></DIV>
<DIV><FONT size=2 face=Arial></FONT> </DIV>
<DIV><FONT size=2 face=Arial>73's Phil....VK6APH </FONT></DIV>
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style="BORDER-LEFT: #000000 2px solid; PADDING-LEFT: 5px; PADDING-RIGHT: 0px; MARGIN-LEFT: 5px; MARGIN-RIGHT: 0px">
<DIV style="FONT: 10pt arial">----- Original Message ----- </DIV>
<DIV
style="FONT: 10pt arial; BACKGROUND: #e4e4e4; font-color: black"><B>From:</B>
<A title=stevezsuzska@iinet.net.au
href="mailto:stevezsuzska@iinet.net.au">Steve Hubbard</A> </DIV>
<DIV style="FONT: 10pt arial"><B>To:</B> <A title=hpsdr@lists.openhpsdr.org
href="mailto:hpsdr@lists.openhpsdr.org">hpsdr@lists.openhpsdr.org</A> </DIV>
<DIV style="FONT: 10pt arial"><B>Sent:</B> Wednesday, March 24, 2010 7:49
PM</DIV>
<DIV style="FONT: 10pt arial"><B>Subject:</B> Re: [hpsdr] Preamplifier
Schematic source</DIV>
<DIV><BR></DIV>***** High Performance Software Defined Radio Discussion List
*****<BR><BR>
<P>
<HR>
<P></P>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 <SPAN
style="FONT-WEIGHT: bold"></SPAN>(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. <BR><BR>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. <BR><BR>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.<BR><BR>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.<BR><BR>73 de Steve, VK5SH (ex VK6ABZ)<BR><BR><BR><BR>
<P>
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