70cm high-pass filter designs
Hi all,
My portable satellite station consists of a Funcube Dongle Pro+ for RX and a FT-817ND for TX. I use a diplexer in front of the dongle to avoid desensing, but still some of the 2m signal is getting in and causing desensing when working V/U birds. It seems to me that a 70cm high-pass filter in the receive chain would help.
Can anyone point me to some easy to build reference designs for this kind of filters? Power handling is not an issue, since this will only be used for RX. I'm mainly looking to something that I could build from things in my junk box, using air core inductors and ceramic caps (if this a sensible thing to do).
73,
Dani EA4GPZ.
A 2-meter low pass filter might help even more since the 3rd harmonic of that signal falls into the 70 cm band. Glen, K9STH AMSAT 239/LM463
Website: http://k9sth.net From: Daniel Estévez daniel@destevez.net To: 'Amsat BB' AMSAT-BB@amsat.org Sent: Wednesday, April 15, 2015 5:07 PM Subject: [amsat-bb] 70cm high-pass filter designs
Hi all,
My portable satellite station consists of a Funcube Dongle Pro+ for RX and a FT-817ND for TX. I use a diplexer in front of the dongle to avoid desensing, but still some of the 2m signal is getting in and causing desensing when working V/U birds. It seems to me that a 70cm high-pass filter in the receive chain would help.
Can anyone point me to some easy to build reference designs for this kind of filters? Power handling is not an issue, since this will only be used for RX. I'm mainly looking to something that I could build from things in my junk box, using air core inductors and ceramic caps (if this a sensible thing to do).
Many thanks to everybody for all the suggestions. I'll have a look at the filter designs.
There are some people that have suggested that the problem might be the 3rd harmonic from the 2m TX, instead of being overload by the 2m signal somewhere in the receiver. I'm also suspicious that this could be the case, but I did yesterday some tests that seem to indicate that the level of the 3rd harmonic is fine.
I tested with different antennas for TX and RX, with about 2 or 3 meter separation between them (my normal antenna is an arrow type antenna, with on coax for the 2m yagi and other coax for the 70cm yagi). Fist I connected the dongle to the 70cm port on the diplexer (and the antenna to the common port). With this setup I got a bit of desense as usual. Then, I connected the dongle to the 2m port on the diplexer. With this setup the desense is very big.
This makes me think that the problem is with the 2m frequency and not the 3rd harmonic, because the distance between the antennas and the 2m port of the diplexer should be more than enough to kill any 3rd harmonic.
However, it's possible that I am seeing two different effects with this: the 3rd harmonic when the dongle is on the 70cm port and overload from the 2m signal when the dongle is on the 2m port.
What do you guys think?
73,
Dani EA4GPZ.
El 16/04/15 a las 00:07, Daniel Estévez escribió:
Hi all,
My portable satellite station consists of a Funcube Dongle Pro+ for RX and a FT-817ND for TX. I use a diplexer in front of the dongle to avoid desensing, but still some of the 2m signal is getting in and causing desensing when working V/U birds. It seems to me that a 70cm high-pass filter in the receive chain would help.
Can anyone point me to some easy to build reference designs for this kind of filters? Power handling is not an issue, since this will only be used for RX. I'm mainly looking to something that I could build from things in my junk box, using air core inductors and ceramic caps (if this a sensible thing to do).
73,
Dani EA4GPZ. _______________________________________________ Sent via AMSAT-BB@amsat.org. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Don't discount the strength of the 3rd harmonic. It doesn't take much to cause problems. Remember, a 3/2ths wave-length antenna radiates as well as 1/2 wave-length antenna and your 2-meter antenna will radiate fine on the 70 cm band. Also, with "normal" coaxial cable, you are going to get radiation from the line. Commercial two-way installations use at least double-shielded cable for things like duplexers and cavities to basically eliminate interference (as much as possible). Having both a high-pass filter for 70 cm and a low pass filter for 2-meters should reduce the problem even more. Glen, K9STH AMSAT 239/LM463
Website: http://k9sth.net From: Daniel Estévez daniel@destevez.net To: 'Amsat BB' AMSAT-BB@amsat.org Sent: Thursday, April 16, 2015 12:41 PM Subject: Re: [amsat-bb] 70cm high-pass filter designs
Many thanks to everybody for all the suggestions. I'll have a look at the filter designs.
There are some people that have suggested that the problem might be the 3rd harmonic from the 2m TX, instead of being overload by the 2m signal somewhere in the receiver. I'm also suspicious that this could be the case, but I did yesterday some tests that seem to indicate that the level of the 3rd harmonic is fine.
I tested with different antennas for TX and RX, with about 2 or 3 meter separation between them (my normal antenna is an arrow type antenna, with on coax for the 2m yagi and other coax for the 70cm yagi). Fist I connected the dongle to the 70cm port on the diplexer (and the antenna to the common port). With this setup I got a bit of desense as usual. Then, I connected the dongle to the 2m port on the diplexer. With this setup the desense is very big.
This makes me think that the problem is with the 2m frequency and not the 3rd harmonic, because the distance between the antennas and the 2m port of the diplexer should be more than enough to kill any 3rd harmonic.
However, it's possible that I am seeing two different effects with this: the 3rd harmonic when the dongle is on the 70cm port and overload from the 2m signal when the dongle is on the 2m port.
What do you guys think?
participants (2)
-
Daniel Estévez -
Glen Zook