Domenico,
Thanks for doing the math. It looks like the AR2 P144VDG and P432VDG would be the preamps to try. I'd still need some good bandpass filtering, of course, so this is a somewhat expensive proposition. It also becomes unwieldy since I have to operate portable. The Icom IC-R10 does not have a robust front end and that much preamp gain is probably going to cause problems. Already, with the AMSAT preamp, I need to add 6dB of attenuation to prevent the receiver from overloading. No idea what that does to the overall noise figure of the system, but I seem to have a better SNR on the signals with the attenuator in line.
My receive system is:
Arrow 3 element beam for 2m / 7 elements on 70cm > 1.83m of RG-58 > an MFJ diplexer > 0.15m of RG-58 > PAR VHF DN152 notch filter > 0.15m of RG-58 > AMSAT preamp > 0.9m of RG-58 > (6dB attenuator on 2m) > receiver
I probably don't need to keep the diplexer in line on 2m (it's there to eliminate desense on Mode J) nor the notch filter in line on 70cm, but it makes it much easier to switch between Mode B and Mode J to do so.
At this point, I'll probably just have to accept what I have. No filtering is going to get rid of the broadband noise and no preamps can overcome the geometry of an urban area.
However, If anyone has some suggestions for a low cost way to improve this system, I'm all ears. Keep in mind that since I go outside for each pass, all the gear has to fit inside a DSLR bag (see my QRZ.com page for a picture).
73,
Paul Stoetzer, N8HM
On 10/3/2013 11:06 PM, i8cvs wrote:
----- Original Message ----- From: "Paul Stoetzer" [email protected] To: [email protected] Cc: "Alan" [email protected]; [email protected] Sent: Thursday, October 03, 2013 6:02 PM Subject: [amsat-bb] Re: AMSAT Broadband Preamp Specs
Alan,
Thanks for the "real world" example. My receiver is an Icom IC-R10, which the specifications say has a 0.25 uV sensitivity on 144 MHz and 0.32 uV on 435. I operate portable, so I'm not using the preamp for the high gain to overcome cable loss, but to overcome the relatively high noise figure of the receiver. The preamp works great for FO-29. When I have a clear horizon, I can hear it down to 1 degree or so with an Arrow antenna. My problem is the horizon (and a military spread spectrum radiolocation system that causes "clicking" noises all over the 70cm band here in Washington).
73,
Paul, N8HM
Hi Paul, N8HM
Just for your information the sensitivity of the ICOM IC-R10 in CW and SSB is :
144 MHz: 0.25 uV for a S/N ratio of 10 dB 435 MHz: 0.32 uV for a S/N ratio of 10 dB
To convert the above sensitivity in Noise Figure (NF) you can use the folloving formula: -6 2 ( Vi x 10 ) x 20 NF = 10 log [ ------------------------ ] +174 dB 10 BW x S/N
where:
Vi = input signal in uV applied over the 50 ohm RX input BW = Band Wide in Hz (2400 Hz for SSB) S/N = Signal to Noise ratio in power ratio = 10
Back to arithmetic and using a scientific pocket calculator you get the following results:
144 MHz NF = 11.2 dB 435MHz NF = 13.3 dB
The above NF are very high and to overcome only part of your problems ignoring all interference in your QTH that requires sharp notch filtering you need front end preamplifiers with at least the following characteristic:
144 MHz : NF= 0.6 dB and gain G = 20 dB with a very high third order intercept point IP3 and very high 1 dB compression point like a preamplifiers of the Norton type circuit.
435 MHz : NF = 0.5 dB and gain G 20 dB possibly with cavity input circuit like that described in my article publisched recently into the AMSAT Journal March/April 2013
I hope this helps.
73" de i8CVS Domenico