On my V/U setup I have a SP7000 70cm preamp. Working FO29(JAS2) my 2 meter signal overloads the preamp, my signal splattering like an adjacent strong station. With preamp off the problem is not there. In testing I found the overload happens in any combination of 70cm 2m frequency pairs. I can eliminate the problem by lowering the tx power, but by doing so I have a hard time copying my own signal. How can I eliminate the 2m sig overloading the preamp while still maintaining adequate power? Antennas m2 436cp30, Gulf Alpha 2m5el, both right hand polarization. I could use some suggestions. Thanks Bill
Bill,
A good discussion of the problem can be found here: http://www.amsat.org/amsat-new/articles/Mode-J/
Alan WA4SCA
I just turn the output power down and all is fine.Don't need much for FO-29.
http://www.amsat.org/amsat-new/articles/Mode-J/
73, Drew KO4MA LM 2332
----- Original Message ----- From: "Bill Bruno" billbruno@earthlink.net To: "Amsat BB" amsat-bb@amsat.org Sent: Wednesday, September 27, 2006 7:04 PM Subject: [amsat-bb] 70cm preamp
On my V/U setup I have a SP7000 70cm preamp. Working FO29(JAS2) my 2 meter signal overloads the preamp, my signal splattering like an adjacent strong station. With preamp off the problem is not there. In testing I found the overload happens in any combination of 70cm 2m frequency pairs. I can eliminate the problem by lowering the tx power, but by doing so I have a hard time copying my own signal. How can I eliminate the 2m sig overloading the preamp while still maintaining adequate power? Antennas m2 436cp30, Gulf Alpha 2m5el, both right hand polarization. I could use some suggestions. Thanks Bill
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I'm sure there are other and perhaps better ways, but in my work (similar but not FO29) I use a single cavity filter. I had WAACOMM build one for me to my spec, but I note they are now out of business. The can (for 2 meters) is about 8 inches in diameter and about 2.5 feet tall. N connector in and a N connector out. I can move my simplex frequency around about 20 - 30KHz before I need to retune the filter.
Yes, this is a brute force and expensive method, but by golly, it sure works well for me. This filter creates a nice, low noise-floor with just about any on-air transmitter in my shack. I loved it so much; I bought another (smaller) filter for 440 as well.
There are other filters out there as well...but the method I've chosen works best by far for me.
Finally, I've found that for any one given situation, an attenuator is as useful as a preamp. It all depends on your total noise figure and link budget. Also, I feel antenna gain is "more better" than a pre or power amp.
In the end, it sounds like that unless you can add gain or a brute force filter, reduced power in your nearby TX is possibly the only other method. But then I'm sure others on this list will tell me I'm full of crap.
Good luck es 73!!
Gary Memory, N7BRJ
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Bill Bruno Sent: Wednesday, September 27, 2006 7:04 PM To: Amsat BB Subject: [amsat-bb] 70cm preamp
On my V/U setup I have a SP7000 70cm preamp. Working FO29(JAS2) my 2 meter signal overloads the preamp, my signal splattering like an adjacent strong station. With preamp off the problem is not there. In testing I found the overload happens in any combination of 70cm 2m frequency pairs. I can eliminate the problem by lowering the tx power, but by doing so I have a hard time copying my own signal. How can I eliminate the 2m sig overloading the preamp while still maintaining adequate power? Antennas m2 436cp30, Gulf Alpha 2m5el, both right hand polarization. I could use some suggestions. Thanks Bill
_______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
There are two potential issues with Mode J: 1. The 2m TX overloading the preamp on 70cm. This problem can only be addressed by reducing the amount of TX energy input to the preamp. That means a. bandpass/highpass filtering on the 70cm side b. increased separation of antennas c. reduction in TX power A 70cm cavity bandpass filter is pretty easy to rig up. I have step-by-step pictures at http://members.cox.net/nx7u/mobile/4x3x5.htm Now the SP7000 has bandpass filtering in it, but I don't know if it's before or after the preamp. If it's not before the preamp, then it won't help the Mode J problem.
2. The 2m TX is transmitting a significant 3rd harmonic (145x3=435, a most unfortunate combination). Similarly, that means a. bandpass/lowpass filtering on the 2m side b. increased separation of antennas c. reduction in TX power
Even if the 3rd harmonic is -60dBc, with a 100W (+50dBm) TX that's still -10dBm radiated--a big signal.
The most prudent (to me) solution is filtering on both ends. When I ran Mode J mobile--where the antennas had no directivity and necessarily were very close together--using the 70cm cavity filter on RX and an ICE 2m BPF filter on TX allowed for no-worries LEO operation. Plus you have the benefits of good amateur practice (filtering TX), and a more intermod-resistant receive path on 70cm. Nearby 460MHz paging/two-way radio systems can really wreck your RX.
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Bill Bruno Sent: Wednesday, September 27, 2006 7:04 PM To: Amsat BB Subject: [amsat-bb] 70cm preamp
On my V/U setup I have a SP7000 70cm preamp. Working FO29(JAS2) my 2 meter signal overloads the preamp, my signal splattering like an adjacent strong station. With preamp off the problem is not there. In testing I found the overload happens in any combination of 70cm 2m frequency pairs. I can eliminate the problem by lowering the tx power, but by doing so I have a hard time copying my own signal. How can I eliminate the 2m sig overloading the preamp while still maintaining adequate power? Antennas m2 436cp30, Gulf Alpha 2m5el, both right hand polarization. I could use some suggestions. Thanks Bill
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Hi Scott, NX7U
I agree with you about both bandpass filtering on the 70 cm side and lowpass filtering on the 2 meter side.
1. I use bandpass filtering on the 70 cm side with a K3PGP passband filter between the antenna and preamplifier input.
http://www.k3pgp.org/432filter.htm
The above filter has an insertion loss of only 0.062 dB at 435 MHz and provides the bonus of adeguate selectivity to -3dB points at 412 and 466 MHz including a very interesting and original preamplifier protection with easy mechanical fabrication.
2. I use lowpass filtering on the 2 meter side with "A High Power 2-meter Filter" by Mike W. Cook AF9Y described in Proceeding of the 1990 Central States VHF Conference.
Unfortunately I was unable to find a description on internet but if someone is interested on it I can scan a send a copy of the article.
The above home brewed lowpass filter has insertion loss of only -0.065 dB at 146 MHz with a return loss of -40 dB
The harmonic attenuation above 146 MHz as measured with my spectrum analyser HP 8555A and tracking generator HP8444A is the following:
2° hamonic 292 MHz -20 dB 3° " 438 MHz - 45 dB 4° " 584 MHz - 55 dB 5° " 730 MHz - 60 dB
An additional advantage of a lowpass filter against a cavity bandpass filter is that a lowpass filter don't need to be retuned moving between 144 to 146 MHz
I use an antenna mounted 70 cm preamplifier with NF=0.45 dB and G=20 dB followed by a postamplifier with NF= 1.35 dB and G= 15 dB
The 2 meter and 70 cm antennas separation is 2.3 meters and using the above filtering no desensitization occurs even testing with 300 watt RF reaching the 2 meter antenna.
73" de
i8CVS Domenico
----- Original Message ----- From: "Scott Townley" nx7u@arrl.net To: amsat-bb@amsat.org Sent: Thursday, September 28, 2006 4:09 AM Subject: [amsat-bb] Re: 70cm preamp
There are two potential issues with Mode J:
- The 2m TX overloading the preamp on 70cm. This problem can only be
addressed by reducing the amount of TX energy input to the preamp. That
means
a. bandpass/highpass filtering on the 70cm side b. increased separation of antennas c. reduction in TX powerA 70cm cavity bandpass filter is pretty easy to rig up. I have step-by-step pictures at http://members.cox.net/nx7u/mobile/4x3x5.htm Now the SP7000 has bandpass filtering in it, but I don't know if it's before or after the preamp. If it's not before the preamp, then it won't help the Mode J problem.
- The 2m TX is transmitting a significant 3rd harmonic (145x3=435, a most
unfortunate combination). Similarly, that means a. bandpass/lowpass filtering on the 2m side b. increased separation of antennas c. reduction in TX power
Even if the 3rd harmonic is -60dBc, with a 100W (+50dBm) TX that's still -10dBm radiated--a big signal.
The most prudent (to me) solution is filtering on both ends. When I ran Mode J mobile--where the antennas had no directivity and necessarily were very close together--using the 70cm cavity filter on RX and an ICE 2m BPF filter on TX allowed for no-worries LEO operation. Plus you have the benefits of good amateur practice (filtering TX), and a more intermod-resistant receive path on 70cm. Nearby 460MHz paging/two-way radio systems can really wreck your RX.
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Bill Bruno Sent: Wednesday, September 27, 2006 7:04 PM To: Amsat BB Subject: [amsat-bb] 70cm preamp
On my V/U setup I have a SP7000 70cm preamp. Working FO29(JAS2) my 2
meter
signal overloads the preamp, my signal splattering like an adjacent
strong
station. With preamp off the problem is not there. In testing I found the overload happens in any combination of 70cm 2m frequency pairs. I can eliminate the problem by lowering the tx power, but by doing so I have a hard time copying my own signal. How can I eliminate the 2m sig
overloading
the preamp while still maintaining adequate power? Antennas m2 436cp30, Gulf Alpha 2m5el, both right hand polarization. I could use some suggestions. Thanks Bill
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite
program!
Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite
program!
Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Hello! With all this 70 cm preamp info flying around, I was wondering if I could change the subject for a moment....
I'd like to receive the new Japanese microsat that is transmitting on 437.???
I have a Hamtronics preamp tuned to 435 Mhz with N connectors on it. I'd probably feed power to it separately while it's on the antenna mast.
My question is this: I have some cable TV hardline gathering dust in the basement. What are the cons for using that to run from the antenna/preamp into the shack? (besides finding connectors for it) Anybody know where I can find connectors?
Thanks!
73 de KM3G Tyler
----- Original Message ----- From: "Tyler Harpster" tyler881@adelphia.net To: "AMSAT-BB" amsat-bb@amsat.org Sent: Thursday, September 28, 2006 1:15 PM Subject: [amsat-bb] 70cm preamp -- with 75 ohm CATV coax?
Hello! With all this 70 cm preamp info flying around, I was wondering if
I
could change the subject for a moment....
I'd like to receive the new Japanese microsat that is transmitting on 437.???
I have a Hamtronics preamp tuned to 435 Mhz with N connectors on it. I'd probably feed power to it separately while it's on the antenna mast.
My question is this: I have some cable TV hardline gathering dust in
the
basement. What are the cons for using that to run from the antenna/preamp into the shack? (besides finding connectors for it) Anybody know
where
I can find connectors?
Thanks!
73 de KM3G Tyler
Hi Tyler, KM3G
You will find informations on how to beat the high cost of connectors for popular "1-inch" 75 ohm CATV coaxial cable reading Ham Radio Magazine october 1979 page 52 to 55
If you cannot find the article I can scan it for you, please let me know.
73" de
i8CVS Domenico
participants (8)
-
Alan P. Biddle -
Andrew Glasbrenner -
Bill Bruno -
Gary Memory -
i8cvs -
Robert Davies -
Scott Townley -
Tyler Harpster