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Message-ID: <fc7eccec0812051220r7b9e3722h345374222dd8d87a@mail.gmail.com>
Date: Fri, 5 Dec 2008 20:20:00 +0000
From: "Andy Talbot" <andy.g4jnt@googlemail.com>
To: rsgb_lf_group@blacksheep.org
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Subject: Re: LF: E field active antennas
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Hi Peter et group.   Sounds an interesting antenna, but for full band
Rx coverage I can't see what a folded resistively loaded dipole has to
offer over the conventional short whip into a high impedance voltage
follower other than strong signal handling, which isn't an issue at
our new operational site.

The commercial antenna we have on order uses a similar design to many
of the amateur designs that have appeared over the years, it is just
better made mechanically. My home made one will just have to survive
on site until it arrives. There really are only a few ways to design
such an antenna, and most work as well as each other - although some
will overload with only modest local strong signals.

The old Naval one it replaces  - and yes, Mal, the Navy use broadband
active whips for HF receive - was a cumbersome heavy mains-powered
thing dating back many decades that used  valves: three in parallel
arranged as a high IP3 cathode follower to cope with adjacent 1kW
transmitters.  Their latest design, I believe, uses power Mosfets to
obtain the same performance.

Andy  G4JNT
www.scrbg.org/g4jnt



2008/12/5 Peter Dodd <g3ldo@ukonline.co.uk>:
> Hi Andy,
> The simplest broadband antenna might be the T2FD (Terminated Tilted Folded
> Dipole) designed by W3HH. This antenna is essentially a folded dipole with a
> terminating resistor in the opposite conductor to the fed one. This antenna
> sacrifices some efficiency for bandwidth, which can be compensated for with
> receiver gain if necessary. The actual size of the antenna and theMM
> terminating resistor value depends on the actual band coverage required.
> I can send details direct if this antenna is a possibility.
>
> Regards
>
>
> Peter, G3LDO
>>
>> Having just had to make an active antenna for HF (for gainful
>> employment-type work, not Am. radio purposes. The commercially made
>> one we've ordered is on five weeks delivery and it was needed before
>> yesterday) I was wondering about a helical element.
>>
>> As the thing had to be resonably lightweight, I made the antenna
>> element from copper tape on 15mm plastic water pipe rather than use a
>> solid copper tube.  Just for a bit of novelty I wound the tape in a
>> helix, but then started wondering if doing that would make any
>> difference to performance.    Normally, helically winding an antenna
>> (rubber duck type at V/UHF) only serves to distribute loading
>> inductance into a short antenna to make it resonate - unlikely to
>> change improve the loss terms at all.  But I did wonder if the added
>> extra inductance, or increased conductor length (not element length -
>> that is 1.2m) would change the performance significantly from a
>> straight tube.
>>
>> Does anyone have any ideas - food for thought if nothing else?  The
>> base amplifier is one I've used several times before for V/LF up to HF
>> based around a J310 source follower running at 20mA with bootstrapped
>> input followed by a 2N5109 emiter follower at Ic = 80mA. I may have
>> published it way-back-when in the amateur press in the 73kHz days.
>> Not the best design judging by some of those published in more recent
>> years, but did mean all the components were to hand for an instant
>> job.
>>
>> Andy  G4JNT
>> www.scrbg.org/g4jnt
>>
>>
>>
>>
>
>
>