Re: Mixed bag with the Air Core Antenna add-on to Tecsun PL310


Chris Knight <chris@...>
 

Jim,

I thought about the slide switch and believe this could be the right answer
for the reasons you mentioned. The MW band is a pretty large swath of real
estate for a single fixed coil to cover. Another good method of implementing
a change of inductance for lower band (and LW) would be the ability to
easily swap coils. I was thinking of using speaker terminals to make it
"plug-and-play"
http://www.radioshack.com/product/index.jsp?productId=2103795 . A person
could wind coils for different portions of the MW and LW bands and swap them
out easily. I'm using this technique with a 0-365pF variable cap in a
plastic project enclosure that can be swapped among loop antennas.

73,

Chris (N0IJK)

-----Original Message-----
From: ultralightdx@... [mailto:ultralightdx@...] On
Behalf Of jim_kr1s
Sent: Friday, January 01, 2010 5:47 PM
To: ultralightdx@...
Subject: [ultralightdx] Re: Mixed bag with the Air Core Antenna add-on to
Tecsun PL310



--- In ultralightdx@..., "Chris Knight" <chris@...> wrote:


I wonder what would happen if you went from 554uH down to 350uH in this
case (PL-310)? If you have already tried it, please let me know. I'm also
wondering what variations of stray PCB capacitance we can expect from these
Chinese manufacturers.

Good analysis, Chris. I think the length of the leads from antenna to board
probably contribute 3-5 pF by themselves. It appears the maximum capacitance
the chip's varactor can achieve is between 325 and 375 pF. Let's say 350 pF.
Scott was saying 270 uH was about the max for the G8. For the sake of
discussion and uniformity across models, let's say 270 uH and 325 pF maximum
C. That combination will resonate at 537 kHz. If you minimized stray
capacitance then, a 330-uH coil as supplied by Tecsun could resonate across
the US MW band. The problem with Tecsun antennas at MW is probably losses in
the core material vs -61, rather than inductance.

At the low end of the band where it can be resonated, there is a clear
technical advantage to using a larger inductance. The reactance of the coil
increases with more turns, more rapidly than does the ohmic resistance. The
"Q" is calculated by dividing reactance by resistance. If reactance rises
faster than resistance as inductance is increased, then a larger coil will
have higher Q. But to obtain the benefit of this Q, the coil must be
resonated by the tuning capacitance. (Reactance and resistance are both
measured in ohms; check Wikipedia or a copy of the ARRL Handbook for Radio
Amateurs at your local library for more info on the differences.)

It doesn't appear that the range of capacitance available will allow a
single antenna coil to resonate on both MW and LW. A 554-uH coil could
resonate down to 375 kHz or so; as you note, however, it won't resonate at
the high end of the MW band. A second coil and a tiny slide switch, to
select one or both coils, could do the trick.

73,

Jim, KR1S

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