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Preamplifier at the antenna ??

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Ian, G5WQ
Sun Jul 20 2008, 03:46PM
Ian, G5WQ
Registered Member #81
Joined: Sun Mar 23 2008, 07:57PM

Posts: 29
christoph ;
in most of our cases on 6m the reason for using a low noise pre amplifier is to have the lowest noise figure you can possibly get in your system .

typically modern tcvrs have anywhere between 2 and 4 db noise figures .

a gaasfet or other type of LNA pre amp will have a noise figure of around 0.4 to 1db .

by turning off the tcvr internal pre amplifier and using an external low noise pre amp you should benefit from an extra 2-3db lower noise floor .

obviously you need low loss feeder and hopefully less than 1db feeder loss in the system .

fitting the pre amp at mast head is the best point to have it because it will not suffer the feeder loss/noise incurred and wont add the loss figure of the coax attenuation to the overall noise figure , but fitting it inside the shack will be of benefit if you have a low feeder loss under 1db .

you will have a slightly higher noise figure this way if a shack mount pre amp is used , but you will still have a substantially lower noise figure than if using just the tcvr and its internal pre amps alone .

of course the best option is a selectable pre amp which has a bypass facility , then you can use both settings to operate with .

sometimes with low noise pre amps they can decrease selectivity and increase the chance of picking up splatter more easily from other stations nearby , but in general low noise pre amplifiers tend to be used for very weak signal working where there arent normally strong signals around to contend with .

however with modern pre amp designs and devices , they are filtered very much better than old designs and the modern devices chosen now have better selectivity and close signal handling characteristics .

the most imprtant thing to have when fitting pre amplifiers is to be able to match the pre amp to the tcvr , ie the pre amps or designs available now all have differeing gain figures , and can vary by 5db difference .

typically a tcvr internal pre amp is around 20db gain and so you are looking to match that with an external pre amp .

the MGF1302 device has somewhat higher gain than many other devices currently used in LNAs and so it can show a little much on your s-meter if you have a reasonably sensistive modern tcvr .

in my IC756PRO3 with MGF1302 i run -6db attenuation when i am looking over the horizon and then when elevated i can switch off the 6db ATT because the noise level is lower .

noise and conditions on 6m do vary with direction/weather and time of year, so you will need to experiement with different headings and settings in order to find the best for your location etc .

if you buy an SP6 pre amp and sequencer from ssb electronics then these have a variable gain adjustment on them which you can adjust to suit your tcvr >pre amp combination [ SP6= 750w power handling ] .

to set the pre amp agaisnt the tcvr all you do is select SSB , switch OFF the internal tcvr pre amps and then switch on the external pre amp , then set everything [pre amp gain ] so the s-meter is just hovering off the bottom of the scale , it is adviseable you set this towards a quiet direction .

for a quick and easy cheap way of checking "noise figure "of your setup you can also use WSJT 4.98 in MEASURE mode and a 50ohm load connected in place of the yagi .
its a lot to explain how to set it all up and test , but basically you need to set the WSJT noise level to ZERO when you have no internal or external pre amps switched on .
then when you switch the external or internal pre amps on you can see a rough measurement of the noise/gain they produce .
you can also check the noise/gain that is introduced when you connect the aerial to the system .

do not expect the lna pre amp to show a higher noise level though, it should show lower noise level but same or better s-meter reading than you see with just tcvr internal pre amps alone .

i have done tests here in my own station of many pre amplifiers and tcvrs and various combinations of them in order to set my station to perform as best it can .

i currently run 3 separately switched receive tcvrs and 3 PCs when i am monitoring or operating EME on 6m .

my homemade combiner along with an MGF1302 pre amp is adequate to supply enough gain to cover all 3 tcvrs plus the loss incurred by using a 3way divider .

just make sure you do not run both the internal pre amp and an external pre amp at same time , thats not what an external pre amp is designed to be used like .

you use either the internal pre amps or an external one alone, not together .

the addition of an external pre amp is about lowering the noise figure of the system so you can hear lower down into the noise floor , its not about making the s-meter sit at s5 all the time .

your s-meter should be sat at or near to ZERO when you have a correctly installed pre amp and the band is quiet .

most external lna pre amps cannot be transmitted through [except the SP6+SEQ] and will require you to have a split/switched receive/transmit coax , but this is easy to achieve if you have a tcvr with an RX antenna port separate from the main transmit socket .

you can use a single antenna feedline , separate it in the shack using a coax relay , then feed one side with the pre amp in series to the RX antenna socket and the other side to the amp or antenna feedline .

set it up so that when you key up the relay latches over into the transmit line and when in receive it goes off and into RX side , use the NC [normally closed ] side of relay to feed down through the pre amp to RX ant port .
connect tx amp input or antenna feedline to the centre port of relay .
connect NO [normally open] port of relay to the output so239 socket of tcvr so that when you key up the relay latches over into the transmit line and isolates the pre amp .

this way if you lose dc voltage to the relay it will show an open circuit to the tcvr rf output and you wont put any rf down the pre amp line .

modern tcvrs wont bother much about an open circuit on the relay if they see one and you wont blow them up as such if you go into transmit and the relay isnt latched over .

the internal swr protection will back the pa stage off .

hope this is of use .

its a lot to go into and write a" quick "reply , i have spent hundreds of hours "playing" with eqpt and pre amps on 6m here at my own station .

i didnt know whether i should put a definitive reply on this thread but i think there is room on here for my reply i hope

73 ian G5WQ .

.

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Christoph, DF9CY
Fri Jul 25 2008, 11:30AM
Christoph, DF9CY
Registered Member #65
Joined: Thu Mar 20 2008, 04:27PM

Posts: 31
Hello Ian,

thank you for your detailed work out here. Great stuff.
I have built a 6m preamp now (my own design) which includes a band filter tuned to 50.150 MHz band width is sufficient for the DX portion of the band. That means the 3 dB limits are on ca 49,9 MHz and ca 50,300 MHz. The video signals on +/-49,75 and +/-48,250 MHz are already attenuated somewhat. I use a ATF type GaAs Fet device and see a noise figure of about 0,8 dB. I use the same device on 2m and 70cm, where I have 0,32 and 0,41 dB here on our HP8970A noise figure meter. Reason for the slightly poorer figure on 6m is the band filter.
The transfer power of the coaxial relais (CX520 type) should be around 1kW on 6m. I have built one box for 2m and 70cm now with both preamps inside and just finishing the 6m box. Supplying the boxes I made exactly as you describe.
Once mounted at the antenna I will carefully compare the situation with/without preamp. ... on 2m I know it is like "some EME" <=> "lot more EME" and on 70cm like "no EME"<=>"a lot EME".

As I said, if the 6m preamp will give me any advantage it will stay there on the tower.

73 de Christoph
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Chris, G3WOS
Fri Jul 25 2008, 02:09PM
Chris, G3WOS
Registered Member #53
Joined: Wed Mar 19 2008, 12:33PM

Posts: 22
I've used a antenna stack successfully now for 18 years without a preamp. I've never believed I've needed one because I've always suffered from high local noise levels and still do somewhat depending on the direction.

However, in the last two Es seasons I've missed three new countries because I could not clearly hear the very weak signals at this QTH. Now I do not know whether a pre-amp would have helped me in those situations but I well remember the feeling that just 'one dB' more might have made all the difference.

So I have decided to build a preamp and will get it up over the winter. It may, or may not, make much difference but at least I know that I've done all I can to maximise my chance of success in the future. Chris
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Chris, G3WOS
Sat Jul 26 2008, 10:18AM
Chris, G3WOS
Registered Member #53
Joined: Wed Mar 19 2008, 12:33PM

Posts: 22
Has anyone any views on the merits, or otherwise, of a narrow bandpass filter in front of a masthead pre-amplifier? Chris
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Johan, ON4IQ
Sun Jul 27 2008, 07:47AM
Johan, ON4IQ
Registered Member #52
Joined: Wed Mar 19 2008, 12:21PM

Posts: 14
Chris,
The preamp will be a usefull tool but I would suggest putting an extra BPF to reduce the risk of overloading your front end. You will have to keep in mind that very often the preamp will give no extra db readability (this is often the case in city environment). As several OM's pointed out, the preamp will have most likely a better NF, however, for some locations the noise floor is that high that a preamp won't help. For eme with elevation its a different case as noise floor drops when you start elevating.
A test to determine your MDS floor will be in order. Easiest is to inject a signal via a 1/10dB splitter into your rig. The -1db being the antenna and the -10 being the test generator. This way you will be able to determine your noise floor.
For my location (rural) I have a 20dB gain preamp 0.5dBNF, with lowpass in front and a comet BPF at the output of the preamp. The total gain is reduced by a switchable attenuator (3dB steps). I'm currently working of a extra BPF which has a better shape a more attenuation out of band.
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Christoph, DF9CY
Mon Aug 04 2008, 10:54AM
Christoph, DF9CY
Registered Member #65
Joined: Thu Mar 20 2008, 04:27PM

Posts: 31
Hi,
I have my homemade preamp up in the tower now. It is working fine. Measured data are F=0,8dB G=18dB icluding all relays and inside N-plugs and N-sockets. For wiring I used RG400 teflon coaxial cable.
Does it give me some advantage ? I only can say: it seems as it does. I have directions with higher noise, where the amp is useless. I tested yesterday at moonset time with listening to W7GJ and W1JJ EME wise with JT65A. My impression is, that there is a little advantage. But I will verify this in the next days.
73 de Christoph
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Ken, G4IGO
Mon Aug 04 2008, 12:08PM
Ken, G4IGO
Registered Member #31
Joined: Tue Mar 18 2008, 08:16PM

Posts: 62
Hi Chris, any little advantage is of use - particulary on such as EME - however it may be that at the end of the day you just feel more comfortable having a preamp in line - if that is the case then be happy.
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Christoph, DF9CY
Sat Aug 01 2009, 08:36PM
Christoph, DF9CY
Registered Member #65
Joined: Thu Mar 20 2008, 04:27PM

Posts: 31
Hi there,
I have a preamp up there now (MOSFET Type 0,95 dB NF measured). That thing last year lived only a few days and went "kaputt" with a near lightning strike. I carefully checked the new one, if it gives me some advantage. I have a few directions, where noise is low. Here (and only here) I can see (and measure) a slight improvement. I have done this with DL4YHF's SpecLab and some weak signals, which were almost not audible. The improvement is around 2 dB at best which is barely audible, but worth having it for EME for example. Where noise is going up just only a little bit, this is lost of course.
Between the antenna feedpoint and the preamp there is ca 25cm of low-loss RG400 teflon cable. It was an easy solution to put the amp right on the boom close to the feedpoint.
73 de Christoph

[ Edited Mon Aug 10 2009, 10:02AM ]
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Christoph, DF9CY
Mon Aug 10 2009, 09:59AM
Christoph, DF9CY
Registered Member #65
Joined: Thu Mar 20 2008, 04:27PM

Posts: 31
Hi all

here is some stuff I put on my website with this preamplifier. Comments welcome.

http://www.df9cy.de/tech-mat/cy50mhz-noise/cy50mhz-noise.htm

73 de Christoph
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Pop, YU7EF
Fri Dec 10 2010, 07:51PM
Pop, YU7EF
Registered Member #1409
Joined: Sat Oct 09 2010, 06:31PM

Posts: 3
Hi Chris

Rather late replay to your post but it's never too late ...

You may consider using AD6IW wide band LNA with pHEMT MMIC SPF5122Z covering 50MHz all the way to 2.3GHz
It has remarkable gain, extremely low noise figure and, very important, very high IP3 .... published in DUBUS this year

Measured IP 3, Gain and NF vs. Frequency

Frequency MHz IP 3 dBm Gain dB NF dB
50 30 25.4 0.6
70 30 25.5 0.5
144 31 25.3 0.37
222 32 24.6 0.36
432 33 22.6 0.39
1296 35 15.7 0.62
2304 37 10.3 0.8
Measured at 3V at 5V > for 1-2dB at 5V > .5-1dB at 5V > .1dB

As I heard, it can be purchased from author for only 25 US$ which is bargain ...

However, unless you want to go for EME and other weak signal modes LNA can be more trouble than benefit

Hope this helps

73, Pop, YU7EF


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