Re: [IRCA] Loop comparisons / suggestions

[Andrew Ikin <andrew.ikin@xxxxxxxxxxxxx>]

The ALA100 was originally designed for the NDB listener. The recommend loop 
circumference of 20m was to provide an approx. loop reactance of 50 Ohm at 
500kHz , this being a reasonable match to the ALA100 input z. Hence at > 
1MHz to the loop reactance is higher  than the ALA100  input z. Whilst this 
may look like a mis-match, the loops radiation resistance is lower with a 
rise in frequency, hence this gives a constant output verses field strength. 
One benefit of having a higher loop reactance compared to the amp. input z 
is that the loop doesn't shunt the amplifiers' feedback as much as a lower 
inductance would. Therefore, there is less degradation in the amplifier 
noise figure. However, this would normally not be an issue with ALA100 
because the overall s/n would be set by the loop. Whereas, this starts to be 
an issue with 1m dia. loops.

Therefore, making the loop larger shouldn't cause a problem. But it may not 
be necessary with the new ALA100LN.

The new ALA100LN  serial Nos. 200 onwards are probably over engineered in 
terms of noise figure. This being approx. 0.33dB at 1MHz with a equivalent 
inductance of a 20m loop. However, this does mean that the new amp. will 
work well even with an 8m circumference loop. Also the new ALA100LN has a 
23dB gain verses 17dB with the old ALA100s

The ALA1530LN/P head amps. are basically the same as the new ALA100LN but 
with a higher gain of 26dB and a lower input z to match the 1m dia. loop. 
The loop/amp. noise figure above 2MHz is approx. 0.2dB or put it another way 
the noise floor is 12dB lower compared to the thermal noise of a resistor.


73

Andrew Ikin

Wellbrook



-----Original Message----- 
From: Volodya S
Sent: Monday, August 27, 2018 5:55 AM
To: Mailing list for the International Radio Club of America
Subject: Re: [IRCA] Loop comparisons / suggestions

Valid point from Chuck.  I've never had an overload problem with any of my
ALA.  The FLG 100, yes.  It needs the 10 dB attenuation, but the ALA 100
works fine, even with my 10 kW CFAX 1070 within 1 to 2 km from my QTH.
Never any overload.   73,   Walt

On Mon, Aug 27, 2018 at 4:51 AM Chuck Hutton <charlesh3@xxxxxxx> wrote:

> I'm not convinced that the impedance mismatch would change enough to make
> a loop bigger than 20m be less effective than a 25m loop.
>
> I wonder whether Andy factored in overload and was saying "bigger might
> cause urban overload problems".
>
> But this is speculation as Wellbrook doesn't really explain the 20m 
> limit..
>
>
> Chuck
>
>
> ________________________________
> From: IRCA <irca-bounces@xxxxxxxxxxxxxxxx> on behalf of Nicholas
> Hall-Patch <nhp@xxxxxxxx>
> Sent: Sunday, August 26, 2018 9:13 PM
> To: Mailing list for the International Radio Club of America
> Subject: Re: [IRCA] Loop comparisons / suggestions
>
> Wellbrook's documentation for the ALA100LN says "The antenna uses a 20m
> circumference thin wire loop to provide optimum s/n. "   Apparently, 
> larger
> loops provide more of a mismatch to the input impedance of the amplifier,
> so one doesn't get the expected additional gain from a still larger loop.
> But they still work well with a variety of loop dimensions as various 
> posts
> attest.
>
> best wishes,
>
> Nick
>
> On Mon, Aug 27, 2018 at 2:41 AM, R. Colin Newell <coffeecanuck@xxxxxxxxx>
> wrote:
>
> > Andy Ikin wouldâve told you that a standard ALA100 is probably at its
> best
> > at a loop of wire 20-30 feet long.
> >
> > Bigger is not always better with these units.
> >
> > Colin Newell - Victoria - B.C. CANADA -
> >
> > > On Aug 26, 2018, at 6:40 PM, Paul B. Walker, Jr. <
> > walkerbroadcasting@xxxxxxxxx> wrote:
> > >
> > > I couldâve made it even larger but I wanted a cross between large size
> > and
> > > manageable   I couldâve done 40-50 feet long.  Who knows what Iâll
> decide
> > > to do when I end up somewhere else
> > >
> > > Paul
> > >
> > >> On Sun, Aug 26, 2018 at 9:39 PM Volodya S <canswl@xxxxxxxxx> wrote:
> > >>
> > >> Excellent dimensions, Paul.  The larger the loop, the more gain you
> will
> > >> have.  Mine are always as large as I can make it!   ....Walt
> > >>
> > >> On Sun, Aug 26, 2018 at 11:44 PM Paul B. Walker, Jr. <
> > >> walkerbroadcasting@xxxxxxxxx> wrote:
> > >>
> > >>> When I land somewhere again where I have a bit of yard space (i dont
> > have
> > >>> hear, but did at my last place) Im reconstructing my Wellbrook
> ALA100LN
> > >> 25
> > >>> foot long by 10 foot tall valleyball net loop antenna that worked so
> > well
> > >>> for me.
> > >>>
> > >>>
> > >>>
> > >>> On Sun, Aug 26, 2018 at 7:42 PM R. Colin Newell <
> > coffeecanuck@xxxxxxxxx>
> > >>> wrote:
> > >>>
> > >>>> Correction: The Wellbrook ALA series is optimized for 100khz to 30
> > >>> MHz.....
> > >>>> not really 10 KHz - although you will hear lots of stuff < 100 KHz
> on
> > >> it.
> > >>>>
> > >>>> 100% satisfied Wellbrook user.
> > >>>>
> > >>>> Colin Newell - Victoria - B.C. CANADA -
> > >>>>
> > >>>>> On Aug 26, 2018, at 7:36 AM, Russ Johnson <radiok3pi@xxxxxxxxx>
> > >> wrote:
> > >>>>>
> > >>>>>
> > >>>>> Good Morning,
> > >>>>>
> > >>>>> I will likely be purchasing a magnetic loop this fall.  I wasnât
> able
> > >>> to
> > >>>> find much on the web in terms of direct comparisons between the
> > >> Wellbook
> > >>>> ALA1530LP,  W6LVP loop and DX Engineering RF-PRO-1B.
> > >>>>>
> > >>>>> Ability to withstand the weather is of prime importance.  While
> cost
> > >> is
> > >>>> always a consideration, of equal importance are performance (of
> > >> course),
> > >>>> durability, product support, ease of installation and use.
> > >>>>>
> > >>>>> Iâd love to hear any thoughts, experiences and suggestions.
> > >>>>>
> > >>>>> Thank you ,
> > >>>>>
> > >>>>> Russ Johnson
> > >>>>> Lexington NC
> > >>>>>
> > >>>>>
> > >>>>> Sent from my iPad
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