FT B57-D Preamp required

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Steve, G4WCS

Fri Apr 25 2008, 11:33AM
just wondering whether a good masthead preamp will be a worthwhile addition to the shack, or is there little benefit at this frequency range ?
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Trev, EA5ISZ
Fri Apr 25 2008, 01:46PM
Trev, EA5ISZ

Registered Member #13
Joined: Tue Mar 18 2008, 01:43PM

Posts: 143
I don't use one myself Steve (although I may well change that) but I have experienced use of a good masthead pre-amp used by Peter G8BCG one of our committee members and also owner of the station we generally use for G0VSM in 6m contests. Frankly, I was, and am, amazed at the difference the pre-amp makes. Its not as if the coax run is particularly long, probably around 100 feet of UR67, but a very significant improvement in sig to noise. We use a quiet site which helps, but still impressive. I cannot give you any detail on the device but if Peter reads this he may be able to add further information.
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Justin, G0KSC
Fri Apr 25 2008, 05:22PM
Justin, G0KSC
Registered Member #64
Joined: Thu Mar 20 2008, 04:10PM

Posts: 49
A point raised in Trev's post abotu is 'quite location' If you live in a built up area and install a pre-amp, it will be likely you will just see an S5 or S7 of noise floor!

JJ
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Steve, G4WCS
Fri Apr 25 2008, 07:18PM
Steve, G4WCS
Registered Member #283
Joined: Fri Apr 18 2008, 11:55AM

Posts: 70
thanks chaps- will hve a play when I get my aerial up.I enjoy the playing about almost as much as operating.

Ive fished out one of my old Ham radio today magazines and found a 3SK88 low noise design with adjustable gain.
Assuming I can still get hold of toko S18 coils ( the 3SK88 is obselete but I have a few knocking around) , Ill put one together and report back.
with the preamp at the masthead and coaxial relays the unit willl be settable to straight through for strong signal E's work etc, and only switched in when needed .
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Steve, G4WCS
Fri Apr 25 2008, 07:50PM
Steve, G4WCS
Registered Member #283
Joined: Fri Apr 18 2008, 11:55AM

Posts: 70
wow- just done a bit of googling- all coils and 3SK88 MOSFETS still available.

Im sure Ive still got the PCB artwork tucked away somewhere. may make a batch up- I have ( or did have) access to a spectrum analyser and tracking gen at work ( assuming its still there) so can set them up on the button
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Ian, G5WQ
Sun Apr 27 2008, 01:21PM
Ian, G5WQ
Registered Member #81
Joined: Sun Mar 23 2008, 07:57PM

Posts: 29
hi steve ;
the first thing you need to have available is the ability to switch off or attenuate the tcvr internal pre amp .

if you add an external pre amp to most modern radios they will just overload unless you can use an internal 20db of attenuation , ie a button to switch the tcvr pre amp off or to put in 20db attenuation .

reason for using a pre amp externally to the tcvr is so you can use a lower noise device than is internal in the tcvr, in order to lower your noise figure .
typically tcvrs have somewhere around 2 to 4db noise figure and by using an external pre amp instead of the tcvr internal pre amp you are using a device with something like 0.4 to 1db noise figure depending on the device used .

the most commonly used device is an MGF1302 gassfet , this has typically 25db gain at 50mhz with a 0.5db noise figure .

it is a little gainy for most modern 6m tcvrs though and you could do with reducing the gain of it by around 5db .

there are some other low noise devices used in commercial and home brew 6m pre amps , BF981 BF964 ATF21186 .

spectrum communications in dorset make in shack and masthead pre amps using a BF964 or simlar device , various types made but mainly 100w or 400w handling versions in ready built or kit form .

they are also variable gain if you use an in shack version , and have rf sensing .

the 400w versions use pcb mount relays but i am unsure if they will actually handle a real 400w of rf for long without burning out and the through swr across them isnt perfect .

need to be carefull with the masthed versions if using an MFJ269 analyser to check swr, it has just enough output not to switch the rf sensing but to blow the BF964 .

if you want to play though they are cheap and easy kits to build and do work

if you want a better quality masthead pre amp then go for an ssb electronics SP6 pre amp plus you need to buy the sequencer that matches it .
this pre amp uses dc voltage injected from the sequencer and fed over the coax to feed and power the pre amp and its relays .
very easy to install and setup once you have worked out how to wire the db9 connector and how to operate the switches on the sequencer .
the sp6 uses an MGF1302 and has a variable gain pot inside the masthead unit which you can adjust to suit your station .

you can set these up in the shack and then take the masthead box and install it remotely at the antenna feedpoint .
these will handle just about 1000w of rf if used with the sequencer but only 250w if used in rf sensed mode .

if you want to go higher in power, ie for EME moonbounce then youll need to start thinking about adding coax relays so you can split the feedline in 2 at the shack end or you will need 2 feedlines for rx and tx use .

most people will use one antenna feedline and then split it in the shack using a coax relay .
then the easy way to get the extra needed switching port is to use the tcvr receive antenna port as well, this will give you separated RX and TX , ie RX ant port and main so239 TX port on the tcvr .

you can fit the pre amp between radio and amplifier .
wire it so the centre port of coax relay is running out to amp input connector .
NC side goes to pre amp> rx ant socket of tcvr .
NO side goes out to the so239 transmit[rf out] socket of tcvr .

it is better to wire it this way because it isolates the pre amp from any rf from the tcvr in the case of loss of dc voltage to the relay .
ok so the tcvr will see an infinate swr this way but its internal circuit will not allow it to make any rf power if you key up and the relay has no voltage on it to latch it over .

just wire the relay coil one side to PTT grounding connection of tcvr and other side to 12v dc .

put a diode across the relay coil terminals with its band pointing back to the dc + positive connection, this will stop the relay staying latched up in receive ,or if you encounter that when running cw mode you find it wont de key .
it will also stop any voltage spikes from the relay coil .

you can buy low noise pre amps from the usa from ARR advanced receiver research , angle linear , downeast microwave and hamtronics , these need to be used with some form of separate relay system to switch them in and out of receive .

the SP6 ssb elecs masthead pre amp requires little more than itself and the sequencer in order to get running, it is however a fairly expensive way of setting up a masthead pre amp , but it does work very well .

i use the sp2000 2m version on 2m eme along with sequencer and it works very well for me with an ic706mk2g tcvr .

i cant run the SP6 on 6m due to fact i run 1500w on eme and would burn the pcb of the masthead unit out if i used one .

i use an in shack pre amp from ARR , the SP50VDG or the P50VDG using an MGF1302 .
SP50VDG has rf switching and bypass which means i can either use the pre amp or just the tcvr depending on whether i am looking over the horizon or elevated towards the moon .

you can also use an rf amplifier which has a built in pre amp , something like the TE SYSTEMS 0552G , these have a low noise pre amp inside them , you can buy from burghardt radio centre in usa ,it will end up costing you 365 sterling , and this is including purchase of amp ,shipping , import tax and vat and customs charges , to your door in 3 weeks .
it only takes 1 week to get to the uk from usa but the customs clearance inside this country takes absolutely ages at mount pleasant and then you have to wait for the royal mail to do their thing too !! .
.
i have bought 2 TE rf amps from burghardt and they are very good to deal with .



i know you posted somehting about an aerial for 6m , and to be honest you need as much as you can physically allow yourself up in the air, and as high as possible .

a single 6m loop will fair better than a vertical or halo if thats all you can get up , but if you can run to even a 2 ele yagi it will improve matters no end for you .

loop is easy to make, just needs 6m of insulated wire cutting, form into a triangle with point at top to hang it from .
split the lower horizontal wire at the centre and place a feedpoint connection .
cut a matching transformer section of 75 ohm tv coax , 58" long , which connects to feedpoint and other end to main feed coax .

loop is bi directional off side of loop , ie as you look through loop .

you can rotate the loop 90degs for east>west or north>south use .

another aerial is the 6m longwire , 4 wavelengths .

1/4 wave [5 feet] of wire one side of a 4-1 balun , then 69 feet the other .
will act like a 3 or 4 ele yagi , with main lobes about 10-20 degs either side of ends if wire .

swr will be low across whole band , just feed from the balun to shack with 50 ohm coax .

the long wire is bi directional off ends of wires .

you can use one of the qtek or watson 4-1 baluns obtainable from waters and stanton etc .

cheap phasing harness can be bought from haydon communications , they have some old jaybeam ones to sell off .

hope this is of use .

73 ian









the average aerial used on 6m is a 5 ele yagi .

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Steve, G4WCS
Tue Apr 29 2008, 09:28AM
Steve, G4WCS
Registered Member #283
Joined: Fri Apr 18 2008, 11:55AM

Posts: 70
Hi Ian,
thanks for the extremely informative response.
Steve
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