For those like me who have no detailed knowledge of the INRAD filters,
except for the brief data on the web page at qth.com, here's the info
they sent me.
INRAD filters for TenTec radios
Product: Bandwidth: Frequency: Shape Factor
#750 400 Hz 6 MHz S.F. = 1.9
#753 400 Hz 9MHz S.F. = 2.2
#751 250 Hz 6 MHz S.F. = 2.1
#752 2100 Hz 9 MHz S.F. = 1.8
#754 2800 Hz 9 MHz S.F. = 1.7
#755 2800 Hz 6 MHz new model due March
#756 2400 Hz 9 MHz new model due April
All of the filters except #756 are 8-pole. The #756 will be a TEN
pole filter. No more data yet for #775 and #756.
I collect filters. I have been building LF band receivers on and off
for years. Some of them have actually worked.
What do the numbers mean ? Here's my 10c worth. The real experts
will no doubt be kind enough to correct my mistakes.
The bandwidths are self-explanatory; to get an idea of how steep the
filter sides are, multiply the bandwidth by the shape factor, to
get the bandwidth at 60 dB down. (The band widths given are at the 3
dB down from peak points on each side of the filter's center
In the case of the #750, the 60 dB bandwith is rather less than 800
Hz. So the filter sides drop 57 dB in around 200 Hz. That is steep.
The wider filters have better shape factors; this is typical because
when you make a narrow filter you inevitably get a trade-off between
shape factor and loss thru the filter. Steep sides and narrow
bandwidth mean more attenuation. Even so the #751 is only just over
500 Hz wide at 60 dB down from the peak so the response drops 57 dB
in around 125 Hz. That is pretty steep too. The loss figure would be
I'm impressed. As a 95% CW man I'd settle for the two 400 Hz filters,
#750 and #753, plus the #756 to replace the stock front end SSB
filter. If I manage to rob a bank before Dayton, I'd like to try the
#751 as well.
But I have to say that I have been very happy with the standard TenTec
filters I already have. "If you can't hear them you can't work them"
... well after 45 years I should know that, but if the signal does
not reach your receiver then you won't hear them, or work them. The
first place to spend money is on the antenna. Then worry about the
I also have a Drake TR-7 with excellent filters; the Omni-V with stock
TenTec filters handles the European BC crud on 40 rather better than
the Drake with its (arguably) better quality filters. Where they all
tend to fall down is front end overload/intermodulation BEFORE the
filters. Phase noise from the local oscillator is also more noticable
if you have really good filters in the IF.
So there is the dilemma; the best filter in the world is not much use
if the front end of the radio cannot handle strong signals. And here
in Europe we have extremely strong signals to fight against. But I
have worked over 350 countries, way over 300 on 40m. I have never
missed one due to the TenTec filters in the Omni-V. I have missed a
few because of zero beat dead-beat operators and I've missed some
because of propagation. The INRAD filters would be great to have and
use. But I could live without them if it meant I could erect a better
antenna system instead.
Maybe I need to rob another bank before heading for Dayton.
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