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Author Topic: Antenna bandpass filter for PSK???  (Read 4464 times)

Posts: 927

« on: June 29, 2012, 12:12:40 PM »

Our club just finished a really nice Field Day.  20 Meters was the money band, with the CW, Phone, and PSK all wanting to simultaniously operated there.  CW and Phone didn't bother each other too much.  But PSK raised the phone radio noise floor perhaps 10 to 15 db (operating about 14.07Mhz with phone somewhere about 14.25 or so).  Looked and sounded like phase noise from the PSK radio.

That got me to thinking:  How about a somewhat selective filter on the output of the PSK radio, centered on 14.070?  The PSK window is only about 3Khz, and if such a filter would knock down the phase noise by 10 or more db up in the 20M phone portion, problem solved (at least for the phone station).  Not sure if a Pi or T bandpass LC filter would work, probably not selective enough?

Perhaps someone has already done something like this?

Posts: 179

« Reply #1 on: June 29, 2012, 12:46:44 PM »

I think that Rick, NQ4I is working to design narrow bandpass filters so he can have a run station and a search and pounce station both working on 20 CW at the same time.
He might be one to contact about how to design such a filter.

John AF5CC

Posts: 6646

« Reply #2 on: June 29, 2012, 03:30:00 PM »

What kind of power were you running, and how far apart were the antennas (and radios)?
It sounds like the front end of the phone receiver was being overloaded by the PSK radio.  You don't need much power with PSK to get through because of the narrow signal.  Did they try lowering the PSK transmit signal?


Posts: 927

« Reply #3 on: June 29, 2012, 07:57:20 PM »

PSK was running about 30 watts.  Antennas were perhaps 250 ft or so apart.  Don't know what kind of radio was being used for PSK.  Phone was an Icom ProII.  The CW station, when on low end of 20, didn't bother the phone too much.  Mostly some clicks.  So would guess that the PSK radio had fairly noisy phase noise.  Noise floor as displayed on ProII jumped some 10 or 15 db, very uniform across phone band. 

Posts: 476

« Reply #4 on: June 29, 2012, 09:26:10 PM »

The PSK window is only about 3Khz,

This notion drives me crazy on Field Day.  BPSK-31 is no more limited to 14.070 than, say RTTY is limited to 14.080.

Spread out people!  It is not illegal to use the VFO!

We had no interference and we were using one antenna on 4 bands simultaneously. See the Quadraplexer thread for details.
« Last Edit: June 29, 2012, 09:28:49 PM by KG6MZS » Logged

Posts: 223

« Reply #5 on: June 30, 2012, 12:31:57 AM »

Assuming the blame is all on the TX side:
If your TX side interrmod products were the issue - then you need to back off the power.
If it's phase noise then the one option, if you can't add a HI-Q filter before the PA, is a something like Clansman SURF unit.

These are Hi-Q tuned filters than will provide both TX filtering and RX protection from co-sited TXs.
The military used them in radio vehicles with several HF nets running.
There are no doubt similar units with really high power handling for shipborne operation.

At VHF I've used  quarter wave coax stub filters on paging TX outputs to notch their phase noise from other systems RX inputs as close as 5MHz away - guess it would also work at HF.

Posts: 17484

« Reply #6 on: June 30, 2012, 10:51:39 AM »

We had a similar problem running 5 watts.  The sideband noise from the PSK station was actually
worse down around 14.010 than up at 14.060.  It wasn't due to overload, as the effect was the
same with an attenuator switched in.  (And at the CW station, the SSB signal didn't affect
reception at all, though it could be heard in the amplified monitor speaker.)

The first step should be to check each rig on the site for phase noise:  any that don't pass
that test get banned under the category of "does not play well with others".  The less
expensive radios are more likely to fall in this category, but each individual rig needs to
be checked, as there is a lot of variation.  One specific IC-706 was particularly bad,
but other types can show similar symptoms.  If someone insists on bring out a rig with
bad phase noise to Field Day, the best use for it is to put it under the wheel of an RV
to level it.  Otherwise such stations should be spaced a distance of at least 1m per mW
from any other stations on the site (and even this might not be sufficient in some cases.)

You also want make sure that all rigs in use have reasonably crunch-proof receivers.
Many of the older solid state rigs (and some tube rigs as well) were rather deficient
in this regard.  Turning off the preamp and/or enabling an attenuator will help.  (This
might be the problem that our SSB station had this year.)

Then make sure that the sound card driving the radio has sufficient bandwidth that it isn't
generating distortion products 10 to 300kHz away.  You may need more low-pass filtering
between the sound card and the rig.  Otherwise you are relying on the stop-band
attenuation of the SSB filters:   if this is 60dB, that can still put an S9 signal into a receiver
100 feet away, which won't be appreciated.

Once you've made sure that the signal itself is as clean as possible, then it is worth adding
narrow RF filters to all the stations.  It may still be difficult to prevent interference to CW
operation up to 14.060.  A helical resonator about 6" diameter and 12" tall could give a Q
of 1000 if properly constructed, and may be a good approach.


Posts: 927

« Reply #7 on: June 30, 2012, 03:27:05 PM »

I was looking at the Chebyshev filter calculations.  If I use a low pass filter (rather than bandpass, which really isn't needed), 7 order, with corner freq of 14.1Mhz and 2db ripple:  The peak just before the fall off is at 14.070.  At 14.190 response is down about 20db.  Maybe about 15db at bottom of phone band.  And much more higher up.  Any comments?
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