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Author Topic: FIELD DAY & BANDPASS FILTERS  (Read 12901 times)
K7NSW
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Posts: 58




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« on: December 20, 2012, 06:12:15 AM »

Thinking about doing FD with a 3 three station setup. Different people coming and going/operating during the event.  Many/most will be new to FD and hf or have little experience.  Each station will have its own antenna.  A club activity.  I think each station should have a bandpass filter on it at all times using the appropriate filter for the band in use.  I am talking about protecting expensive receiver front ends.  Will be running at 100 watt level into resonant/trimmed dipoles so swr will be low.  Bandpass filters: Good idea?  Important thing to do? What do you think?
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N7SMI
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Posts: 315




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« Reply #1 on: December 20, 2012, 06:36:12 PM »

Our field day site didn't have bandpass filters last year and while receiver protection will be of some concern, it is cross-signal interference that the filters will really help. We could really only use one station at a time due to all of the interference.
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K0IZ
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Posts: 737




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« Reply #2 on: December 20, 2012, 08:07:03 PM »

At our club we had four stations going.  No problems with too much voltage to receivers.  But phase noise was a problem, which bandpass filters would help (when rigs are on different bands).  Unfortunately at least two of the stations, for example CW and SSB, would both want to operate on 20M during day, and that caused a fair amount of cross-interference.  Prior years that problem had been much worse but our antennas were further apart this year.
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K7NSW
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Posts: 58




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« Reply #3 on: December 21, 2012, 08:18:43 AM »

Here is one way to do it.  Three xcvrs: we operate on 20 40 & 80.  Three dipoles on one pole up 50 feet center fed by one coax: 120 degrees spread around the center pole.  Coax goes to triplexer.  Triplexer feeds the three xcvrs, each with a bandpass filter.  Power level is 100 watts ssb.
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N4NYY
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Posts: 4742




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« Reply #4 on: December 22, 2012, 04:29:39 PM »

We have used Ice filters and their effectiveness are hit and miss.
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K0IZ
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Posts: 737




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« Reply #5 on: December 22, 2012, 05:37:25 PM »

Another possibility is coaxial stubs.  These are usually 1/4 wave length (adjusted for velocity factor of coax).  RG6 works fine.  A 1/4 wave length, shorted at end, is basically an open circuit in that band, and more or less a short on other bands.  A 1/4 wave length, open on end, is basically a short on that band.

I made up a box with a switch and several lengths of RG6 coax.  We used at field day.   One of the radios had fairly high phase noise, so the box was connected to that radio.  The coax's tended to reduce phase noise on the other bands.  I measured between 10 and 15 db (mostly 15) of improvement.  Not as good as bandpass filters, but RG6 is cheap.  You could make up a simple one-band filter with one length of coax and a T connector.
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AE5QB
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Posts: 267




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« Reply #6 on: January 01, 2013, 11:16:32 PM »

Filters are the way to go.  While stubs are cheap, they are not band pass filters.  They are tuned stubs which means they are resonant on one freq with a fairly high Q. They will not give you good service across an entire band.  Filters, on the other hand, are designed as band pass devices, they have a much wider bandwidth.  We use Array Solutions filters and they are a good company.  For under $400 your club can have a complete set.  Check them out here:

http://www.arraysolutions.com/Products/as-bpffilters.htm

One word of caution.  You need to really control your operators.  Transmitting on the wrong band with a filter installed can destroy the filter.  Make sure that the operator of each xcvr knows what band they can operate on.  Also, this doesn't take care of the issue of trying to operate multiple radios on the same band i.e. one SSB and one CW. 
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K0IZ
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Posts: 737




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« Reply #7 on: January 02, 2013, 04:10:20 PM »

Tom, I agree with you that stubs are not equivalent to bandpass filters.  I found 10 to 15 db improvement, noticeably less than good bandpass.  But junk RG6 costs nothing.  In our case, with antennas fairly far apart, the stub improvement was adquate, and priced right.
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WB6BYU
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Posts: 13038




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« Reply #8 on: January 03, 2013, 01:33:04 PM »

I'm planning to build a set of filters for the club this year:  we had problems with
interference between stations last year running QRP, and they want to use 100W
this year.

We've used them before with other clubs, and they seemed to work quite well,
even to prevent interference between CW and phone on the same band. The
real problem will be what to do with the digital station:  I think a cavity resonator
that tunes from 14.065 to 14.075 might do the trick.

However, the radios still have to be up to the job.  That's one problem with some
of the inexpensive and/or all-in-one-box radios:  they often have poor dynamic
range and/or bad phase noise.  Turning the preamp OFF is a good start on the
former, but many hams aren't willing to acknowledge that their pet radio splatters
all over the spectrum.  We had to haul out a spectrum analyzer to show one FT757
owner how bad his rig was, then he borrowed the analyzer to show his friends how
bad THEIR rigs were.  He wasn't as popular after that.  (And it is particularly difficult
when the club president or Field Day chairman is generating the worst phase noise.)

Anyway, checking the rigs beforehand is important, and knowing how to set the
front end controls to handle strong signals.  Assuming that your rigs are capable of
reasonable performance on both TX and RX, then adding filters in the antenna leads
can make a usable solution, along with minimizing coupling between antennas and
eliminating unwanted antenna resonances on other bands.  (When we used a mono-
band yagi on 20m we would remove the element tips from the tribander we used
on 10/15m to reduce the amount of 20m RF it picked up.)
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W5ESE
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Posts: 550


WWW

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« Reply #9 on: January 04, 2013, 10:24:15 AM »

I built a set of bandpass filters for our club's FD; from the Sept 1988 QST. The article is available on the ARRL's web page under:

http://www.arrl.org/rf-and-af-filters

They work well, although there are some newer designs that are better.

I got the parts from 'Dan's Small Parts and Kits', and they were inexpensive to make.

It can also help a lot to turn on your transceivers' attenuators and dial down the RF gain.
 
Using single band antennas helps, too; and provides some intrinsic "filtering".
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M0HCN
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Posts: 473




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« Reply #10 on: January 05, 2013, 03:17:54 PM »

This actually begs a question:

How much of the PN is down to the low power stages in a transceiver as opposed to the stages running at more then say 20dbm?

If most of the noise is down to the LO and low power stages could not a pair of relays be added to switch the receive bandpass network in between the mixer buffer and the pre driver on transmit?
You want to end up with the filters as late in the chain as possible, consistent with the drive power not being excessive.

This would effectively eliminate the phase noise contribution from the low power stages leaving only the noise inherent in the medium and high power amplifier stages, which if they are starting at +20dbm or so is not likely to be overly significant.

Seems to me that doing the work at low power usually trumps doing it at high power, particularly as the rig likely has suitable filters already.

Of course if the PN is coming from an overly noisy PA strip then to do any good filtering must be at the high power output, but I have a hunch that is not usually the case.

73 Dan.
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WB6BYU
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Posts: 13038




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« Reply #11 on: January 05, 2013, 07:03:11 PM »

Phase Noise really needs to be eliminated in the VCO by good design and component
selection.  It affects both the transmit and receive chains.  Unfortunately the
narrow bandwidth required to filter it out is less than the bandwidth over which the
VCO needs to be able to operate, which makes frequency filtering impractical.

Yes, additional switched filtering in the early stages will reduce the impact on other
bands, but that is contrary to the prevalent design philosophy of making a wide
band HF rig with sub-octave switched filters on the output.
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M0HCN
Member

Posts: 473




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« Reply #12 on: January 06, 2013, 05:45:54 AM »

I never quite saw the virtue in that approach, why let more get to the first mixer on receive then you have to, and if you are going to filter on receive before the first mixer then surely using the same filter on TX only makes sense (You might want to switch in a buffer between the mixer and filter to terminate the mixer out of band)?

Agreed that LO phase noise can only really be dealt with at source, and that some of the very wideband VCOs and synthesizers leave a lot to be desired, the saving grace here is the move to DDS which (if you use the good stuff) makes the dominant close in PN component that of the crystal/SAW oscillator. I will take a few low level spurs over PN any day as the spur requires you to get unlucky with a strong carrier, while PN will mix with everything out there.

Still seems to me that doing transmit BPF 40dB earlier in the chain makes more sense then doing it at the 100W level.

73 Dan.
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KO7I
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Posts: 106




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« Reply #13 on: January 13, 2013, 08:33:42 PM »

This is what we did last year (W7PIG 3A WWA, 3rd place, 7th call area, 20th overall). 2 Towers. One tower dedicated to CW/DIG and seperated from the SSB tower by 250 ft.
The SSB #1 (20M & 75M) & SSB #2 (15M & 40M) had a tri-plexer and shared a Tri-band yagi. The CW/DIG station had a tri-band yagi and a fanned 40M/80M Inverted vee.
Last year 15M was marginal and 10M was dead. I did spend a couple hours Sunday on 15M.
We used the Dunestar BP filters on the tri-plexer and they worked out very well.
20M had a IC-756 proII and 15M had a TS-590. For 40M we had a 40M BP filter on the SSB delta loop and on 75M we had a 75M BP on that stations 75M inverted vee.
To do well, our main run stations SSB #1 and CW needed to be operate 20 meters at the same time. While the BP filters did not help us with "in band" interference, the 250 ft seperation between towers made the difference. The CW station was running a Omni VI.
Going with a tri-band yagi and having stations operating of 20 & 15 or 15 & 10 is a sure fire way to have a great time at Field day. If you choose to run a GOTA station on 10M when 20m SBB is on the air you have to choose each respective operating frequencies wisely (ie 20M 14.225 and above & 10M 28.450 and below).
Building the tri-plexer from the QST article is super easy, fun and satisfying.
73,
Don KO7i
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