*JFET op amp vs bipolar op amps, *LTSpice simulations connected to a 3
meter monopole
A bipolar op amp doesn't always give the lowest noise with a short monopole
at 1.8 MHz because the op amp current noise creates noise voltage across
the antenna capacitive reactance. Additonally, op amps can be connected in
parallel for lower voltage noise at the expense of higher input capacitance
(which loads down the antenna signal voltage).
Simulations of the YCCC amp with a bipolar op amp and a JFET op amp were
performed. Next, preamps having 2, 4 and 6 JFET op amps were modeled. As
the number of op amps goes up the antenna voltage goes down with optimum
S/N ratio, in this case, with 4 op amps. It shows 4.5 dB better S/N than
the bipolar op amp. The bipolar op circuit is modeled with an LTC6228
rather than an AD8045 because I didn't have to import the AD8045 model into
LTSpice. I can try to import the AD8045 later if anyone is interested. The
LTC6228 op amp has 1/3 the voltage noise and the same current noise with
Bias Current Compensation disabled, so it should show lower noise than the
AD8045. The JFET op amp is the ADA4637 and IMD simulations were run with it
looking better than the bipolar.
*YCCC amp with bipolar vs JFET op amp for a 3 meter monopole at 1.8 MHz*
LTC6228 bipolar op amp, 9.5 nV/Hz^0.5, loaded sig 0.93, relative S/N ratio
0 dB
ADA4637 JFET op amp, 6.7 nV/Hz^0.5, loaded sig 0.85, relative S/N ratio 2.2
dB
2 X JFET op amp, 4.8 nV/Hz^0.5, loaded sig 0.74, relative S/N ratio 3.9 dB
4 X JFET op amp, 3.6 nV/Hz^0.5, loaded sig 0.59, relative S/N ratio *4.5 dB*
6 X JFET op amp, 3.1 nV/Hz^0.5, loaded sig 0.49, relative S/N ratio 4.2 dB
Dave KH6AQ
On Wed, Mar 11, 2020 at 1:49 AM Chris Moulding <
chrism@crosscountrywireless.net> wrote:
> As well as being a radio amateur (G4HYG) I also run a small business
> designing and making radio equipment (Cross Country Wireless).
>
> Recently I've been asked by a radio contest group to see if I can
> redesign the YCCC Hi Z amplifier using modern components and using
> similar mounting arrangements to our Loop Antenna Amplifier.
>
> The first prototype using surface mount components is working well.
>
> So far I've not build an array of antennas but that will come later when
> the production boards arrive.
>
> The prototype uses a unity voltage gain amplifier and a BNC connector.
>
> I've a couple of questions for others on the list with experience of
> running vertical receive arrays:
>
> Is a unity voltage gain amplifier OK or do you think it needs more gain
> for long coax runs?
>
> At present I'm using a BNC male connector for the output. Would an F
> type connector be more compatible with existing antenna arrays.
>
> 73, Chris G4HYG
>
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> Reflector
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