Pre amp isolation

[VK2CMP]

Good evening all,

I have just completed setting up my 23cm setup hopefully for a career in QRP EME. I am running 100watts (250 PEP) into two 55 el loop yagi's (21dBi each) with a minikits preamp up the mast. The yagi's sit 9m above the ground above my house (90m ASL) to clear the house etc.

Question in the process of interfacing amps, sound cards, PCs, radios, mic etc the preamp seems to have bit the dust (shame really as it requires the tower to be lowered..). I did a few calcs (always good after the event0 and worked out the preamp was probably soaking up about 0.5 watts on TX. The relay I use is a CX800N which gives about 27db isolation at 23cm. I was going to install two low power relays and terminate the preamp into 50ohm on TX. Does this make sense to those who have been down this path?

cheers
Mick
VK2CMP

[VK3DXE]

Hi Mick,

I think you're looking for something like this: http://www.dg2kbc.de/img/seq/rxtx_schematic.pdf

I'd be looking at the loss characteristics of the relays as the prime focus, but the info in the link above should do the trick for you. It is a tried and trusted method.

[ZL1RS]

I did a few calcs (always good after the event0 and worked out the preamp was probably soaking up about 0.5 watts on TX. The relay I use is a CX800N which gives about 27db isolation at 23cm.

Yeow, 0.5W into the preamp. Look no further! Actually, 27dB down from 100W carrier is 0.2W, but the effect will be the same.

27dB isolation is no where near enough ... typically looking for 70dB isolation or more here. There are several makes of coaxial relay that will achieve this on their own, but a smaller relay to terminate the preamp into 50 ohms on RX as shown in the link in VK3DXE's reply is a good idea for more than just the isolation. It also allows informative receive system measurements by being able to compare external noise sources (e.g. the sun) to the fairly consistent noise from the 50 ohm termination. On my old 2m system (4 x 12 ele Yagis) in a quiet location it was even possible to tell the difference between noisy and quiet areas of the sky. When the moon was in an area of "quiet sky" the noise would actually be 1/2 a dB less on the antenna than on the 50 ohm load and EME reception was markedly better ... so quiet you could "hear a pin drop"!

73, Bob ZL1RS

[VK4CZ]

Whilst you could design a time delay generator as shown in the schematic, I've recently used a locally produced version that's available from Minikits incorporating four switched outputs in a a cost effective solution....

http://www.minikits.com.au/electronic-k ... /Sequencer

[VK2CMP]

Hi Alan, Bob and Scott,

thanks for your suggestions. Yeah tough life being a preamp.

Interesting design. At this stage I just have the one coax (LDF 550) up the mast for TX/RX with DPDT relays at the pre amp and amp. However I will effectively be implementing a very similar approach using two of the minikit relay kits (http://www.minikits.com.au/electronic-k ... 6GHz-Relay) that I built up over the weekend. I was going to put one at the input and output port.They have 60db isolation so with the 27db in place I will be sitting at 87db and all should be good.

I also built and tested over the weekend the new design mininkits preamp and was going to change that over as it has a lower NF than the old design.

I did use the minikits sequencer in my old amp and bang for buck it is great. For this amp I used the DEM kit as it just is a bit more flexible as it offers a DPDT relay for every output so you can lash up 12 v/ 24v/ or GND etc at easily.
http://www.downeastmicrowave.com/PDF/trs.PDF I think the kit was about $30.

Like most aspiring EME'ers I'm sure I'll be running a different antenna and more feeds up the mast in no time to good to know what to aim for. Hey I nearly have the XYL agreeing to a 2.4m dish on top of the house!

seeya
Mick
VK2CMP

[ZL4DK]

I would strongly reccommend running a second coax up the mast for a recieve line. Since this will be on the output of your preamp it doesn't have to be super low loss. If you don't you will need two high-power/high-isolation relay which as you have found is difficult to achieve at 1296 and it is common to use a high power relay and low power relay pair to achieve this.
There are also some rules about what length of coax you should use between these relays to give the best performance as far as isolation is concerned. Sorry I can't point you in exactly the right direction here, I suggest you give google a try.

Sorry just read your post again and saw you were using DPDT high power relays. Yes that avoids the need for two high power relays. I see you will use a pair of low power relays either side of your preamp to give the isolation to both sides. You will probably want to arrange the control wiring on these so you can switch the input one seperately to the 50ohm load so that you can make antenna/resistor noise comparisions as Bob ZL1RS mentioned.

Regards
David ZL4DK

[VK2JDS]

as soon as you can - change away from those loop yagis to as big a dish as you can organise with a septum feed (google ok1dfc septum)

yagis will allow you work the big guns, but being linear you are already 3dB behind the rest of us.. everyone except rare dx expeditioners uses circular pol

I was working a loop yagi station on the german base in antarctica when he went and adjusted the aim and cracked the ldf5-50 at -40C... that finished the amp off and he went sk after that

he was decodable with his about 400watts at the yagi, i run a 5 metre dish and 130 watts but he was struggling to decode me till the feedline broke....

keep the loop yagis for terrestrial play, when you get serious is circular pol and a big dish. the CW this weekend was great on 23cm, from 1296.010 to 030 it was packed, 040 had the big swiss station HB9Q parked on it and was easily worked. Doug vk3um had parked on 016 and was 55n as well.

you will really enjoy 23cm, but dont think terrestrial, EME is dishes and circular pol, G4DDK lna's and No cable from feed to relay to LNA, just adapters

always willing to help, Dave vk2jds

[VK2CMP]

Hi David - I use the DPDT relays so I can easily switch the PA or Preamp out of circuit when working strong signals. I have enough control wires (4 feeds and 4 pwr/ control for LNAs at present) up the mast to put in an additional function. Good idea to be able to terminate the LNA as you describe David - however would still like an arrangement which allows me to bypass it as well. I'll work out a plan for the present setup and rig (IC910H) to migrate over time to the separate RX/ TX. Lots to do

Hi Dave- I like the idea of a dish its just my neighbours don't understand.. I spent nearly two years with council and various experts to get a 13m Nally and triband beam up in the back yard, so am conscious of not stretching the friendship with the mast on the house (see pic). Its 9m above the ground and can be seen miles away. Comments include howz the radar station going and do you work for ASIO. Was thinking of getting a black van with tinted windows and parking it out the front!

I'll stick the new lower noise preamp at the top of the mast this week and was also thinking of changing the 23cm yagi's to stack side by side to improve the signal to noise due to the noise picked up from the ground even thou this will make the beam width in the hoz direction sharper.

The switching at 100w all looks good as I did some tests on JT65 and the amp and switching all looks good. I could just see a coupe of traces but no decodes yet - must admit still learning WSJT. I had the new preamp set up in line at the rig over the week end so I'm hoping when I swap out the old U/S unit with the new on and the additional relays things will improve

Then it is what improvements to focus on next. I'm thinking at this stage that any new antenna will be at the expense of one or more of the existing beams. If I go a dish it would only be a 2.4m. Plenty to think about and research which makes this mode so much fun. I'm sure I'll have a heap more questions.

Thanks for the comments so far all

cheers
Mick
VK2CMP

[I2NDT]

you can also try the "front-end protection device" described by K3PGP in 1977...
http://www.k3pgp.org/432filter.htm : its isolation is said to be around 40dB.
it can be scaled for other frequencies as well.
it must be installed between the antenna change-over relay and the preamplifier.

i2NDT Claudio

[VK2CMP]

Giday Claudio,

interesting solution and website thanks for the info. I wonder if thats what ssb electronics use in their power protector?

http://www.ssb.de/product_info.php?info ... -band.html

I've nearly installed a 2nd relay now, just bought a bigger box because I need more real estate to complete it.

ciao
Mick
VK2CMP

[I2NDT]

I don't know that SSB device...
BTW I remember I used a 1/4 lambda piece of coax cable (actually 1/4 lambda x VF of the coaxial cable) and 2 back-to-back diodes to replace relays in my VHF FM PA.

i2NDT Claudio

[VK2CMP]

Hi all,

I have installed the low power isolation relays on the preamp that terminate it into 50 ohms. As per David and Bob's suggestion I have run a control line to enable me to terminate into 50 ohms in RX. Thanks for the suggestions. I also swapped out my old minikits preamp for the new version that has a lower NF.

Took a bit longer than planned. Seems every time I lower the mast it buckets down for days on end.. Should have done this when the big dry was on a while ago.

Cheers
Mick
VK2CMP