Re: [Repeater-Builder] GLB Preamplifier - Preamp Device (transistor) Information
At 1/11/2009 06:12, you wrote: Correct, here is the order info sheet from the wayback machine 73, Joe, K1ike OK, is it just me or is there NO noise figure spec. in any of these documents? My guess is that the GLB preselector consists of the filter followed by the preamp. Also because of the number of sections of filtering the overall small size of the unit, the actual helical filter is probably quite lossy. For comparison the 5 section helical resonator in the front end of the G.E. Mastr II/MVP VHF HB radio has about 6 dB of loss. If you need preselection and low noise figure, I'd look here: http://anglelinear.com/custom/custom.html or do what I do look at the hamfests or eBay for one or two 1/4 wave bottles followed by the preamp of your choice. Pass cavities are typically 0.5 to 1 dB loss. The Angle Linear cans are even lower - I measured 0.25 dB on one of his 220 MHz filters. Bob NO6B
Re: [Repeater-Builder] GLB Preamplifier - Preamp Device (transistor) Information
skip I opened up My 2 meter GLB preamp preselector. it is an MRF-901. Years ago there was an article in one of the Ham radio handbooks that showed a 4 element UHF preamp/preselector that I built for ATV use. and it too was based on an MRF-901. It worked great. GaSfets I feel have too much gain, and introduce too much noise for repeater use. If Im not mistaken the Kendercom receivers use 2 MRF-901's in the Front end. Other than Commercial Gear(motorola/GE) I felt the Kendercoms were the best receivers out there. the newer Maggiorees are not bad either. Depentds on what eles is on your site that can create havic. Neal --- On Sat, 1/10/09, skipp025 skipp...@yahoo.com wrote: From: skipp025 skipp...@yahoo.com Subject: [Repeater-Builder] GLB Preamplifier - Preamp Device (transistor) Information To: Repeater-Builder@yahoogroups.com Date: Saturday, January 10, 2009, 2:24 PM Re: GLB Preamplifier - Preamp Device (transistor) Information Hello again Sailors! The smoking lamp is now lit... smok'em if you got'em (just a trip down memory lane if you get the above circa 1940-1945 reference). Anyway... per my previous post requesting anyone who wanted to part with (sell or trade) a GLB Receiver Preamplifier unit (any band, any condition)... I did actually receive an original GLB and an Aria-GLB (Simrex?) unit to reverse engineer. Dealing with part one... it appears the GLB preamplifier active device in the original unmodified unit I have here is actually a Dual Gate Mosfet. The Mosfet has only UG printed on one side of the device but the circuit is an interesting take on a Mosfet layout and I was able to pretty much confirm the device in the circuit I have here is a DG Mosfet. Credit where it's due... the original GLB design I'm currently working on appears to be a well thought out and practically engineered product. Going back through the group archives... I see some posts that suggest and/or confirm the device in their preamp was reported to be an MRF-901 (mrf901) device. I tried to review the GLB Pictures in the photos section of the group... but they dont' appear to load/show up. Joe, if you still have those photos, would you please resend them direct to me. I should be able to confirm the circuit is different or the same to the original GLB Preamp I have here. So... drum roll please... I suspect the original Dual Gate Mosfet device to be similar to or the actual BF981 Mosfet, which of course is no longer in production. I have found a substitute device (in current production) with greatly improved IMD performance... so a next step of this project would be to figure out the best practical retrofit. More to follow... If anyone has copies of the original GLB Preamp photos I don't seem to be able to view or download from the group... please send them to me at my regular email address below. I'd like to sort out the MRF-901 device being actually used in some pre-amplifiers or just an educated guess. thanks a mucho... cheers, skipp skipp025 at yahoo.com Yahoo! Groups Links
Re: [Repeater-Builder] GLB Preamplifier - Preamp Device (transistor) Information
skipp025 wrote: Re: GLB Preamplifier - Preamp Device (transistor) Information This is how I remember it. From GLB, any preselector-preamp built below 200 MHz used the Bi-Polar MRF-901. You could, however, have any frequency preamp built with a GaAs FET if you paid extra for the option. I don't know what the GaAs device was, as I never had to replace one. Kevin Custer
Re: [Repeater-Builder] GLB Preamplifier - Preamp Device (transistor) Information Part 2
What is the NF of a stock UHF Micor/Mitrek/MASTR II? Anyone ever measured it? My experience has been that 16 dB gain provides a noticeable improvement, but a low NF is the real key to success. If the preamp gain is higher than 20 dB, it's easy to put an attenuator between the output of the preamp and input of the receiver to bring the gain down to about 16 dB. In the preamps I built, I put a resistive pi-network attenuator (-3 to -10 dB depending on the active device) right in the preamp enclosure. In addition to reducing the gain, it also enhanced the stability of the preamp with respect to changes in load impedance. my two cents worth, not counting inflation, Eric KH6CQ --- On Sat, 1/10/09, n...@no6b.com n...@no6b.com wrote: From: n...@no6b.com n...@no6b.com Subject: Re: [Repeater-Builder] Re: GLB Preamplifier - Preamp Device (transistor) Information Part 2 To: Repeater-Builder@yahoogroups.com Date: Saturday, January 10, 2009, 5:25 PM At 1/10/2009 14:01, you wrote: A first glance and test of the BF981 indicates/measured a relative noise figure value of .6dB to .7dB, which is actually a fair value better than I would have thought for an older Dual Gate Mosfet circuit. Although a Gasfet would have a lower (better) noise figure... would it really be practical to retro-fit (modify) an original GLB Preamp for the difference in noise figure performance. I don't yet plan to ramp up an MRF-901 preamp for noise figure comparisons. ... but I might. The datasheet for the MRF-901 shows ~1.5 dB NF @ 450 MHz. Expect at least 2 dB NF out of the preamp, more if there's a preselector ahead of it. Nothing wrong with all the Gasfet gain if you can keep a handle on it (tame the tiger). One ham out here on the left (west coast) gets away with series GasFet preamps separated only by a 1dB pad. It's red hot, seems to be stable and works like gang busters... not to mention the mountain top is not super clean. So I have seen nearly 30dB of preamplifier layout behind good enough generic T1500 type band pass filtering work fairly well in actual applied operation. Nice to have all that gain if you can get away with it... But there's no reason to use to use that much gain unless your RX is very deaf to begin with (worse than 0.5 µV for 12 dB SINAD). 20 dB gain from a theoretical, noiseless preamp would bring a 0.5 µV RX down to 0.05 µV, which is slightly less than kTB (lower than you can measure with a 50 ohm sig. gen.). I find that the 16 or so dB of gain afforded by the Angle Linear preamps is the perfect amount of gain for G.E. Mastr II/MVP/Exec II RXs, probably the Micor as well. Bob NO6B
