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Band Segment Filters for Transceivers on Same Band, Different Modes

Feb 6th 2013, 05:38

WB6FRZ

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
I'm looking for a low to medium cost solution for an age-old problem. Two stations operating the same HF band (call it 20m) with 100W each at opposite ends of the band: one on CW, the other on SSB. Without spending $5,000+ per radio for super-selective rigs how does one avoid or seriously attenuate interference? Yes, cross-polarized antennas help but I want a solution that can be built by any amateur radio individual/group for, say, Field Day, or any multi-operator event.

Notch Filters- maybe. But how to design one with a 3dB notch bandwidth of 100 kHz or so and 90 dB of attenuation? I know if it was easy it would have been done by now. There are lots of sharp minds out there - any thoughts?

Thanks.

73 de John, WB6FRZ
Feb 6th 2013, 14:24

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Hi John WB6FRZ,

I built a 40 meter filter that measured 6x6x12 inches that would allow good attenuation of SSB when you are operating CW and vice versa, but although I published the design in QEX, I have never heard of anyone going through the trouble of doing all the sheet metal work and such to build another--perhaps because it was lossy receive only filter--an RX/TX filter would need to be even larger!

In theory, if you were to take a pair of those transmitting loops and couple them together, you could make a very narrow bandpass filter. But, it is a lot of expense and effort to do that. On the other hand, if your club did a group buy, and used those loops as main station antennas when not using them in multioperator contests, perhaps the expense might not be so great.

You might investigate the idea of building ultra narrow bandwidth antennas. Maybe one could run an array of transmitting loops? Computer control might be able to handle the phasing.


Zack Lau W1VT
ARRL Senior Lab Engineer
Feb 15th 2013, 13:53

K8GU

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Do you have experience with this interference in past operations or are you trying to do your homework for a future operation?

Operating in-band at 100 watts is not difficult at all in my experience if you have modest (say 300 ft on 20 meters) antenna spacing and opposite polarizations. The big problem is if one of your transmitters produces a lot of broadband hash (most do) that raises the noise floor on the receiver at the other end of the band. No amount of receive filtering will fix that and transmit filtering would be tricky.

Elecraft (and I think Ten-Tec also) have worked very hard to reduce broadband hash on transmit. This is one of many things that make the K3 a popular DXpedition radio. I'm not sure what the status of other companies' efforts are. I know from experience that the FT-857, IC-706, and IC-7000 are noisy.
Feb 27th 2013, 05:08

WB6FRZ

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Zack, thanks for your thoughts.
In which issue of QEX is your filter design published?
I suspected that this project might be quite large, so size doesn't scare me; bring it on.

I am working with our club "antenna guy," Brian, N6IIY to maximize the benefits of cross-phased antennas spaced apart as well as filtering concepts.

Thanks for the help.

John, WB6FRZ
Feb 27th 2013, 05:17

WB6FRZ

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
K8GU, thanks much for your thoughts.
Our radio club has experienced in-band cross-mode interference for years and for some reason I suddenly have a desire to eliminate the problem. It has plagued us during Field Day operations for as long as I can remember. So the answer to your question is yes, and yes.

Thanks for the note about broadband hash; I didn't realize many radios are that poor. It may be that what I'm trying to accomplish is not possible due to the broadband noise of many of our transmitters.

Very 73,


John
WB6FRZ
Feb 27th 2013, 10:53

W1VT

Joined: Apr 4th 1998, 00:00
Total Topics: 0
Total Posts: 0
Nov 1998 - QEX (Pg. 58)
A Narrow-Bandwidth 40 Meter Band-pass Filter (RF)
Author: Lau, Zack, W1VT

QEX is not available for download, but reprints are available via Anthony Nesta, our reprint specialist, at 860-594-0390--he takes credit card numbers. Also via anesta@arrl.org. $5/per article/per issue, postpaid. Works part time, mostly in the afternoons.

Big, low loss filters should also be effective in reducing broadband noise, if used on both transmit and receive. You will need to solve issues with high current and heat--melted solder joints and arcing are to be expected unless your design addresses these problems.

http://www.jenningstech.com/pdf/cap_cat1.pdf
Not all vacuum variable can handle 100 amps of current--be sure you aren't exceeding the ratings of your capacitors.

http://hamwaves.com/antennas/inductance.html
This online calculator looks interesting--I'd be interested in any reports on how useful it is in designing very high Q coils.

http://www.wa4dsy.net/filter/filterdesign.html
This calculator does both notch and bandpass filters. But, it doesn't account for the finite Q of the parts.

Handbook of Filter Synthesis by Anatol I. Zverev
This is the classic book with tables that do account for the Q of the parts.

Zack Lau W1VT
ARRL Senior Lab Engineer

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