Hi all…

I've recently purchased a 2m linear amplifier kit to replace a FL-2025
linear that keeled over on me some time back. The set I'll be using
it is a Yaesu FT-290R II and I intend to run the set in low power mode
(~300mW as best I can tell) to reduce the size of any attenuators in-
circuit.

In fact, I intend to make the attenuators switchable; probably a two-
port device using SMA connectors, so I can switch to a 5W attenuator
if I want to use a handheld into it, or even take it out completely if
I plug in a homebrew set of 100mW power.

The power amp module I'm using is the Mitsubishi RA60H1317M, which has
a maximum input power limit of 100mW. I've been told 50mW is plenty
enough to saturate the output, so likely, I'll be aiming for no more
than 50mW. By my reckoning, a 10dB attenuator should give me 30mW.
I'm only allowed 30W FM on my grade license, and the set previously
only did 25W, so if it's that rather than the full 60W, I'm happy.

I am allowed to use up to 100W SSB, and maybe I might look into eeking
out the full 60W PEP there, but that'll be a later project.

One thing I wish to ensure though, is that should the set accidentally
get switched to full power, or swapped for another set, that something
*other* than the amplifier module or transceiver, blows up. If I
smoke an attenuator, big deal. I'd like to ensure however that the
fault can be quickly detected and suitable protection is in place.

I'm thinking along the lines of voltage clamping with diodes. Working
backwards; 100mW RMS into 50 ohms, gives 2.236Vrms. Converting this
to a peak-to-peak voltage, we have 3.162V, which means a voltage swing
from -1.581V to +1.581V.

This seems close-enough to the 1.4V forward voltage drop that's
typically seen when combining two 1N4148 diodes in series. Two pairs
of series-connected diodes in an anti-parallel configuration, would
divert the RF power when the peak-to-peak voltage exceeds 2.8V. This
translates to an RMS voltage of 1.980V, which across 50ohms gives
78.4mW.

Safely within the amplifier's limits. I'm wondering what the catch
is. It looks simple, but I notice people don't do it, which makes me
think there's a reason why this is a bad idea, aside from the fact the
diodes are going to distort like hell. (And lets face it, the amp
will distort if you overdrive it anyway…)

Apart from use of appropriate attenuator circuits, and making sure you
check power levels at all times, is there anything I can do to prevent
such accidents?

Regards,
Stuart Longland VK4MSL

Stuart Longland VK4MSL wrote:

Hi all…

I've recently purchased a 2m linear amplifier kit to replace a FL-2025
linear that keeled over on me some time back. The set I'll be using
it is a Yaesu FT-290R II and I intend to run the set in low power mode
(~300mW as best I can tell) to reduce the size of any attenuators in-
circuit.

In fact, I intend to make the attenuators switchable; probably a two-
port device using SMA connectors, so I can switch to a 5W attenuator
if I want to use a handheld into it, or even take it out completely if
I plug in a homebrew set of 100mW power.

The power amp module I'm using is the Mitsubishi RA60H1317M, which has
a maximum input power limit of 100mW. I've been told 50mW is plenty
enough to saturate the output, so likely, I'll be aiming for no more
than 50mW. By my reckoning, a 10dB attenuator should give me 30mW.
I'm only allowed 30W FM on my grade license, and the set previously
only did 25W, so if it's that rather than the full 60W, I'm happy.

I am allowed to use up to 100W SSB, and maybe I might look into eeking
out the full 60W PEP there, but that'll be a later project.

One thing I wish to ensure though, is that should the set accidentally
get switched to full power, or swapped for another set, that something
*other* than the amplifier module or transceiver, blows up. If I
smoke an attenuator, big deal. I'd like to ensure however that the
fault can be quickly detected and suitable protection is in place.

I'm thinking along the lines of voltage clamping with diodes. Working
backwards; 100mW RMS into 50 ohms, gives 2.236Vrms. Converting this
to a peak-to-peak voltage, we have 3.162V, which means a voltage swing
from -1.581V to +1.581V.

This seems close-enough to the 1.4V forward voltage drop that's
typically seen when combining two 1N4148 diodes in series. Two pairs
of series-connected diodes in an anti-parallel configuration, would
divert the RF power when the peak-to-peak voltage exceeds 2.8V. This
translates to an RMS voltage of 1.980V, which across 50ohms gives
78.4mW.

Safely within the amplifier's limits. I'm wondering what the catch
is. It looks simple, but I notice people don't do it, which makes me
think there's a reason why this is a bad idea, aside from the fact the
diodes are going to distort like hell. (And lets face it, the amp
will distort if you overdrive it anyway…)

Apart from use of appropriate attenuator circuits, and making sure you
check power levels at all times, is there anything I can do to prevent
such accidents?

Regards,
Stuart Longland VK4MSL

How about an automatic PINdiode attennuator. I've seen one in a magazine.
You might find with Google.

Regards,
Jan PE1SBG

On Jul 10, 11:02*pm, "Jan Jongejan, PE1SBG"
wrote:
Stuart Longland VK4MSL wrote:
Apart from use of appropriate attenuator circuits, and making sure you
check power levels at all times, is there anything I can do to prevent
such accidents?

How about an automatic PINdiode attennuator. I've seen one in a magazine.
You might find with Google.

Regards,
Jan PE1SBG

Ahh okay, that's one possibility… I could conceivably default to
fairly high attenuation, then ramp it up until it hits the desired
mark.

http://www.rf114.com/lib/download.ph...e=bseq&seq=695
is one such circuit I dug up. I'll have to delve into the theory a
bit more to see if I can make it work.

Thanks