Clansman SURF 25W
Description
The SURF 25W is an optional band pass filter that connects between the
RT321 and either the TURF-25W or a 250 Watt Amplifier. It has a set of 8
of switched bands and a variable tuning control. Note that the output is
not connected (and there is at least 60dB isolation between input and
output) when the Tune/Operate lever is in the Tune position. The SURF
is used to improve rejection of other strong local signals at sites with
several HF stations. It provides at least 18dB rejection at +/- 10% of the
operating frequency at the expense of up to 2.5dB insertion loss.
Operating
The SURF 25W is an optional component of the MEL RT-321/322 system.
If in use it should be connected between the RT-321 and TURF-25W in a 321
or between the RT-321 and Amplifier 250W in a 322 installation using BNC
patch leads.
There is really no need for a SURF unless operating close
to other HF stations and it should be in direct mode if fitted but not
needed, since it noticeably reduces the power to the antenna even if
accurately tuned.
If it is needed to deal with strong signals nearby or to improve harmonic
rejection then proceed as follows:
- Set the SURF band switch and set the RT-321 frequency dials
- Power up the RT-321 (saves a lot of bleeps if left until now!)
- Put the radio and SURF into TUNE mode
- Adjust the SURF tuning control for highest meter reading on the 321
- Switch SURF to Operate/Lock and proceed to tune TURF.
Images
Technical Notes
Internally the SURF25W is formed of two ganged series tuned circuits between
the input and output BNC sockets. The Band switch selects one of 8 tuned circuits
and the DIR position (DIR for DIRect) connects the input and output directly
when the slightly lossy SURF is not needed. The Tune control operates a ganged
set of variable capacitors. The LOCK/TUNE switch operates a brake on the capacitor
shaft and also connects a dummy load in place of the output when in tuning mode.
Note that the SURF is designed specifically for use with the RT-321 and uses the
RT-321 meter to indicate correct tuning. This is based on the RT-321 seeing best
SWR when correctly tuned.
EMERs
The SURF-25W is described in EMER H592 (circuit) and H594 part 2 section 3 (Test Spec).
Frequency Response
These images are frequency response traces prepared using a Wiltron 6407 scalar
network analyser in two port mode. Please note that they are in order starting
with direct (pass through) and then from highest to lowest range, with the tuning
control in mid position for all of them and the sweep is 1 to 45MHz for most of
them, except the 3 lowest ranges which are 1 to 10MHz. Vertical scale is 10dB/Division.
This example seems to manage better than 20dB rejection at +/- 10% of carrier.
Best rejection is around 60dB at twice the carrier (2nd harmonic) but gets worse
with increasing frequency to around -30dB at 45MHz.
Click on the images to view full size
Using SURF25W with other transmitters
It has been suggested that the SURF25W could be used as a harmonic filter to add
25-30dB of harmonic suppression to older transmitters that would otherwise be unwise
to use under current commercial radio standards.
The SURF25W should be usable with any 50 ohm coaxial output transmitter up to the
power rating of its internal dummy load. An inline SWR meter will be needed to determine
tuning or the dial will need a calibrated scale adding with the aid of an RT-321.
This approach has been confirmed by Ed G8YXR using the SURF as a harmonic filter for a
Wireless Set No. 19.
Last Updated
Thu Oct 20 23:28:18 BST 2016