On Tue, 31 Mar 2009 20:37:58 -0700, Joerg
wrote:

JIMMIE wrote:
On Mar 27, 12:43 pm, Joerg
wrote:

From vague memory I think Geloso used them a lot and got a little under
4dB out of those on 144MHz. In case anyone needs a datasheet:

http://www.r-type.org/pdfs/6cw4.pdf

According to this page
http://www.amsat.org/amsat/archive/a.../msg00451.html
Geloso got 3.6 dB@50/144 MHz to 6.0 dB@432 MHz.

The VHF numbers are still usable at terrestrial VHF communication due
to the high band noise. For satellite and EME communication, the
performance it not very good at VHF and practically useless on UHF.


Tinkering with the nuvistors also sounded like fun. Well I now realize
why you dont see projects based on them and noone is scouring the
tables at the hamfest looking for nuvistors. There are better/easier
of ways of skinning this cat. A DC to Daylight preamp using a MMIC
may well have better NF and gain.


True, but tubes and even those tiny ones do have one distinct advantage.
When a thunderstorm rolls through town or you accidentally transmit into
your pre-amp their chance of survival is orders of magnitude higher.
Just imagine, when placing a hard 50V spike onto the grid a tube won't
even flinch. A GaAs-FET or MMIC? Tsssk ... *BANG*.

Most part of the lightning electromagnetic radiation is at quite low
frequencies (below 1 MHz) and a typical VHF/UHF antenna will have a
quite low effective height at such low frequencies, thus not much low
frequency energy should enter the preamp.

However, during thunderstorms, quite large static charges can be
accumulated in ungrounded antenna elements and if there is a DC path
to the transistor, there is a significant risk of damage.

Using a folded dipole and grounding the center point will reduce the
static build up. The folded dipole will also act as a very small
magnetic loop, reducing the LF pickup of the lightning EM pulse. A
strong DC path (such as a shorted 1/4 wavelength stub) from the
amplifier input to ground will also reduce the DC and low frequency
pickup, thus reducing the risk for dangerously large voltages at the
transistor base/gate.

Paul OH3LWR

Paul Keinanen wrote:
On Tue, 31 Mar 2009 20:37:58 -0700, Joerg
wrote:

JIMMIE wrote:
On Mar 27, 12:43 pm, Joerg
wrote:

From vague memory I think Geloso used them a lot and got a little under
4dB out of those on 144MHz. In case anyone needs a datasheet:

http://www.r-type.org/pdfs/6cw4.pdf

According to this page
http://www.amsat.org/amsat/archive/a.../msg00451.html
Geloso got 3.6 dB@50/144 MHz to 6.0 dB@432 MHz.

The VHF numbers are still usable at terrestrial VHF communication due
to the high band noise. For satellite and EME communication, the
performance it not very good at VHF and practically useless on UHF.


Although back in those days UHF was reserved for the more well-heeled
hams. I thought the 2m performance that old John got out of the
nuvistors was pretty good.


Tinkering with the nuvistors also sounded like fun. Well I now realize
why you dont see projects based on them and noone is scouring the
tables at the hamfest looking for nuvistors. There are better/easier
of ways of skinning this cat. A DC to Daylight preamp using a MMIC
may well have better NF and gain.

True, but tubes and even those tiny ones do have one distinct advantage.
When a thunderstorm rolls through town or you accidentally transmit into
your pre-amp their chance of survival is orders of magnitude higher.
Just imagine, when placing a hard 50V spike onto the grid a tube won't
even flinch. A GaAs-FET or MMIC? Tsssk ... *BANG*.

Most part of the lightning electromagnetic radiation is at quite low
frequencies (below 1 MHz) and a typical VHF/UHF antenna will have a
quite low effective height at such low frequencies, thus not much low
frequency energy should enter the preamp.

However, during thunderstorms, quite large static charges can be
accumulated in ungrounded antenna elements and if there is a DC path
to the transistor, there is a significant risk of damage.

Using a folded dipole and grounding the center point will reduce the
static build up. The folded dipole will also act as a very small
magnetic loop, reducing the LF pickup of the lightning EM pulse. A
strong DC path (such as a shorted 1/4 wavelength stub) from the
amplifier input to ground will also reduce the DC and low frequency
pickup, thus reducing the risk for dangerously large voltages at the
transistor base/gate.


The trouble in my case was that this thing was also used for the upper
shortwave bands, when doing duplex transmissions. Similar to what is
nowadays called an active antenna. I didn't have the space for a 2nd
large antenna, the first one was already quite a stretch. This was in
Germany where properties are quite tiny.

--
73, Joerg