Beginner question - complete system

[M0YDH]

Thanks Dave. I have 2N2907 and 2N3906 pnp transistors. May I use either of these?

I ordered Mini-kits pre-amplifier For 70cm and a bias tee to power it plus a 99p buck converter for the coax relay. A DL6WU antenna for 70cm and 23cm bands will be under construction with lots of encouragement from Dr John Cooke GM8OTI who wrote about practical versions in PW. I might have a DATV QSO in 2018 especially if my neighbour G7MEG's beleaguered Portsdown gets working!

Perhaps there are a few more units to go on the system diagram?

73
David M0YDH

[G8GKQ]

Hi David

Both those transistors should work, although given the choice I would go with the 2N3906.

It would be great to have ALL the information on the Wiki, but there's only so much content that the core Portsdown team can post and keep up to date. We rely on the wider community to fill in the bits round the edges! So please experiment and let others know how you get on.

Dave

[M0YDH]

Yes Dave. I do put findings and pictures of gear on the Wiki. If only to prove that designs are reproducible. I'll have a go with sequenced switching by Portsdown.

[M0YDH]

Forgive me but I remain confused because I've never built a vhf or higher station with masthead pre-amplifier. No need for these exists in Summits on the Air activations! Height ASL and no electrical noise make 2m and upwards easier.
I thought the objective was to use one coax line between the station and the masthead units? If I have a 4 port transfer relay like on the Wiki at Masthead do I install another coax relay say SPDT in the amplifier enclosure (screened box ?) ? Where does the bias tee go in all this? Screened box and after the low pass filter?

Cheers
David M0YDH

[g0mjw]

You can use one coax and a transfer relay to switch in and out a pre-amp Make sure you sequence it properly and power off the pre-amp before the relay switches and allow the contacts to settle before applying RF. You will still need TX/RX switching at the bottom. I would save that relay for higer frequencies and on 2m build a DG8 pre-amp for 2m. It has the filtering in it you will need.

Mike

PS - no pre-amp explains why SOTA stations are often a bit deaf. Most 2m rigs have NF over 4 dB need a pre-amp in a quiet location to get the optimum sensitivity.

[M0YDH]

Thanks for the note Mike. I remain confused. Where in relation to other units does the bias tee normally go? I had the comment at club that after the LPF is unusual but do put it in its own screened box.
The DG8 uses two DPDT relays in a way that is eminently simple - Pre-amp in circuit on Rx and bypass for Tx. I've a couple of those relays but was told that 146MHz was there practical limit. When DG8's become available again I'll build the kit for my 2m station. I'm reading the notes on it tonight as these are recommended in the Wiki.
I'm building a 70cm system first then a 23cm system when I've learned what to do. I'm struggling to see how to connect a 4 port transfer relay for pre-amp in line on Rx or bypass for Tx. The wiki article on changeover relays uses the fourth port for a dummy load which looks very sensible. Are there always two relays at masthead or which bit of information is missing?

I also have a SPDT coax relay and a 5 port SMA which I think is a rotary and even more of an application mystery!

Please may I have pictures and a good comprehensive diagram update? The one earlier in the article here seems to show 2 coax lines and no system to control masthead activity at the relay or relays.

Sorry if I come across as thick. I'm only a chartered mechanical engineer so skiing well off-piste.

cheers
David

[g0mjw]

I still don't know why you are looking at a Bias T. Its not required unless you want to send DC up the coax for some purpose. You don't need one. I suggest you forget about it for the moment to avoid confusion.

The Transfer relay has 4 ports, J1-4. Typically the relay toggles between J1 is connected to J2 and J3 to J4 - Call this state TX and J1 is connected to J3 and J2 to J4, call this RX. (Note it could alternatively be J1-J2 and J3-J4, vs J1-J4, J2-J3 - measure it with a continuity tester)

You could connect your Transceiver to J2 and your antenna to J1. Your preamp between J3 and J4.

In the TX state the TRX (J2) and Antenna (J1) are connected directly. So you can transmit through it.

In the RX state the antenna port J1 connects to J3, and J4 to J2, so the signal from the antenna (J1) goes through your preamp (J3 in, J4 out) and as J4 is connected to J2 it gets to the TRX.

The only problem with this is the the TX state the preamp input (J3) is connected to its output (J4). Therefore it is quite important it is powered down or it will oscillate, potentially self-destructively. I wouldn't really recommend this in case of mistakes, but it can be done. Normally pre-amps use two relays to avoid this sort of danger, and also because transfer relays are rarer and more expensive.

This is not the only way you can use a transfer relay. You can use just 3 ports, like a normal relay, or as in the Wiki. You could use it to switch in and out an power amplifier or a filter instead of a preamp.

See here perhaps

https://literature.cdn.keysight.com/lit ... 9:epsg:dow