Most people know coax has loss, the higher the frequency the greater the relative loss. You can go buy better coax with less loss – however every coax has loss and once the signal is lost, it’s lost.
This is more prevalent on receive as in reality loss in the receive system equates to noise, and loss before a preamp is added as noise in the preamp gain along with the wanted signal, not what you want. On transmit you can over come coax loss by increasing the transmit power at the radio end of the coax.
The way to get round the receive issue is to put a preamp as near the antenna feed point as possible so there is as little loss, thus noise, added to the wanted signal. In reality with a preamp at the antenna feed point the gain of this amplifier only needs to equal that of the coax losses, so if you have coax losses of 10dB then you don’t need 30dB of gain!
In the early days of contesting in the G3PYE/P contest group 23cm was always our weak point. We done what we could to minimise our losses between the radio and antenna. The target was reducing the loss of received signals as well as increasing the transmit power to our antenna so we were heard by other stations.
Usually at G3PYE/P we use 25m of Ecoflex-15 coax on the 20m pneumatic mast, but on 23cm we found even this length of decent coax had a big restriction on our signal and we could manage a couple of dozen QSOs tops. Now the Ecoflex-15 datasheet states for 25m of coax the loss would be 2.45dB (100m = 9.8dB), in reality we found our 25m runs to be nearer 6dB! this was on 2 different lengths of Ecoflex-15 we have from different drums… So for 23cm we cut a run to 10m which was 2.4dB and ran the mast at 8m which improved our losses but impacted our coverage due to less height above ground. For further info we use a homebrew 47element yagi, we made 4 of them for the 2009 VHF NFD.
As part of the improvements to the G3PYE/P station contest group I build a masthead mounted preamp to overcome all coax losses on receive, this is nothing special, however was I also done was to build it in a box which also housed a 45w transmit amplifier. Not so common. Actually I built 2 identical units, one for G3PYE/P and one for G6UW.
The preamp was an LNA one supplied by Sam G4DDK and the transmit amplifier was one of John G4BAO’s.
If we looked at our average positions in the RSGB 23cm UKAC contests then before we started using the 23cm masthead system we had an average position of 4.4th place (5 events) in restricted section. After starting to use the masthead system our average positioning became 1.3 over 6 events, a huge improvement.
The System is In 2 Parts
Here is the block diagram of the system.
As you can see there are 2 parts to this, the Masthead Box and the Control Box. The Masthead Box is, quite obviously, mounted up top next to the antenna. The control box sits with the radio and between them is 3 core mains cable, as thick as you can get so there is as little voltage drop as possible and the coax feed.
Notice in additional to that the 24v line in the control box is such that the preamp control MUST be off before the TX control line can goto 24v on the control line – this is the interlock mechanism to ensure both the preamp and TX amp can be enabled together, which would likely destroy the active device in the preamp.
I’ll cover the Control Box first as it’s simple and can be covered quickly. Basically the TX control is done by hard-switching a relay, connected to the radio this input line is grounded on PTT to enable 24v on the control line.
RX is basically a reversed bias-T which is used to drive a relay from the 12v the Icom IC-910 puts on the coax for powering a normal masthead preamp but instead of powering the preamp it energises a relay in the control box which switches 24v on the control cable when the IC-910 has the preamp enabled. I use the IC-910 to drive the preamp control as the IC-910 will always remove this 12v from the coax before going into TX which protects the preamp.
The masthead box houses the active transmit amplifier, the receive preamp and relays to switch each in or out. The amplifier is one designed by John G4BAO and is based on an MRF9045 28v 45w device. 2.5w in is all that is needed for the max output. This is ideal as the Icom IC-910 has 10w output on 23cm and by the time it gets to the top of the Ecoflex-15 with it’s 6dB loss we have the voila 2.5w needed, perfect.
Here you can see the TX amplifier on the top cover with the preamp in the middle below and the relays either side of it. You can also see with 30w out here (2.0w in) for 3.34A @ 26v, 86.84 DC watts, which shows 34.4% efficiency, the datasheet claims 41% so quite close showing the design is good. When the amp is at 2.5w in the efficiency rises to 37.6%.
The preamps has 38dB gain with 0.28dB NF which is amazing really. Possibly too much for terrestrial applications, and I did plan to put a 10dB attenuator on the preamp output to stop overloading the Icom IC-910 receiver, not done that yet but in the UKAC’s we don’t see an issue. The connector for the DC is a 3 pin waterproof plug is a Bulgin Buccaneer type which is good to 10A.
To mount the TX amplifier, the heatsink was glued to the outside of the box and a hole cut where the PA would attach to it. The PA board is screwed to the heatsink so in effect sandwiches the box lid between the heatsink and the PA box so it won’t move out of place.
To waterproof the heatsink in order to stop water getting through the hole we have made in the lid, I used liquid tape on the outside edge of the heatsink.
So I not only made one of these units, I also made one for G6UW, the Cambridge University club, they are effectively clones of each other.