Introduction
The GH Quad was initially designed as a solid state alternative to the commonly used 2C39 valve amplifiers used for DX and contesting at 1296 MHz. It has since been modified to cover the lower part of the band used for ATV. It was intended that the Quad would sit on a bench in the shack or operating position and that the DC supply would be from a standard 13.8V PSU.
However, there has been a considerable amount of interest in mounting the Quad at the masthead, with a view to reducing the feeder loss to a minimum. This poses a number of issues which will be mentioned below. GH Engineering has no experience of mounting the Quad at the masthead, and as such does not recommend doing so. However, it is fully appriciated that the amateur radio licence is issued to encourage 'self-training' for those interested, and that new ways of making direct point-to-point contact (even if via a repeater) should not be discouraged. It is acknowledged that some users will remote mount the Quad anyway, and so this information is supplied in order to reduce any problems that may occur.
These notes are therefore given to those who wish to experiment and
persue ever new routes and methods of radio and television communication.
They are presented because a number of features have been incorporated
into the design of the Quad, and these features should be taken into account
if attempting to mount and use the Quad at a remote location such as at
a masthead. These notes do not form any kind of warranty or guarantee
whatsoever.
At this point in time, GH Engineering is unable to give specific advice on any mechanical or electrical arrangements that may be required in order to remotely mount the Quad. However, any feedback or comments on remote mounting will be collated and published on the web site so that others can benefit, unless specifially requested to the contrary.
GH Engineering can assume no liability for loss or damage, whether personal
or otherwise, howsoever caused, by the use or attempted use of the Quad
as a masthead amplifier.
Remote mounting the QH Quad
There are a number of issues that must be taken into account very carefully
if the Quad is to be used remotely. In no particular order, these
include :-
1) Mechanical stability
2) Weatherproofing
3) Power supply
4) Cooling
5) Remote monitoring
6) Drive requirments
1) Mechanical stability.
Some means of attaching the Quad to a mast will have to be devised. This attachment will have to be fairly substantial, and it may be advantageous for this attachment to bolt directly to the heatsink. However, see the notes on cooling in section (4). The attachment will then have to be fixed or located on the mast with a suitable clamp. The standard heatsink weighs over 3kg, When the Quad is mounted in a suitable case, the total weight will be approximately 4kg. Depending on what type of cables are used, and how they are mounted, the likelyhood is that the attachment itself will weigh at least 0.5 kg, thus giving a total weight of approximately 5kg.
The weight alone will not be a problem for a standard 2" aluminium or steel scaffold pole. However, unlike mounting a large yagi aerial, which will be mounted at, or very close to it's point of balance, the Quad and it's associated mounting mechanism will exert a bending moment on that mechanism, and in particular the clamp. If the Quad is mounted next to a mast, with the longest side of the heatsink touching the mast, then a bending moment of at least 0.5kgm will be exerted on the mounting clamp. This is the 'best case' condition for bending the bending moment, and in practise the bending moment could be cosiderably greater.
One way of reducing this bending moment is by the use of a counter-balance,
which would in itself add more weight depending on the type of counter-balance
used.
2) Weatherproofing
There are two possible applications for remote mounting of the Quad; one is for temporary use such as contesting, and the other is for permanent installation. These require quite different criteria when weatherproofing is to be cosidered.
A temporary installation is likely to be for no more than 48 hours, and most contesting (and all portable contesting) takes place between March and October, thus avoiding the worst of the winter weather. Therefore some means has to be devised of keeping out rain for a two-day period. There is also the issue of condesation; This occurs every night - the gas molecules in the air contract, and are not able to keep molecules of water in suspension. Consequently, water droplets form at the coolest places, which will be the walls of the amplifier case. The effect of condensation on the electronic components within the Quad is uknown.
A permanent installation has to endure the worst of the weather, including
constant drizzle, fog, driving rain, snow, hail and sometimes some bright
sunshine! It is appriciated that mast-head pre-amps are in reqular
use in thousands of locations around the world and can provide reliably
good service in all weather conditions. However, mast-head pre-amps
are very much smaller and ligher than power amplifers, and they do not
generate considerable amounts of heat. Some means must be found to
weatherproof the case, and the connections for the RF input/output, DC
input (and output if required - see section 5) and PTT input if required.
These problems are by no means unsurmountable, but still they must be given
careful consideration.
3) Power supply
The Quad requires a DC supply of nominally 13.8V at around 18 amps. The main supply voltage is applied to the Vcc pins of the modules at all times. There is a solid-state PTT function which turns on the bias supply to modules when on transmit. This arrangement has the advantage that no relays are reqired to carry large amounts of current; only the bias current is switched which is in the order of 3A. The PTT current required to switch the bias is around 4mA.
A reduction in the supply voltage will have no adverse effect on the amplifier other than reducing the output power by a small amount. However, an increase in the supply voltage will reduce the life of the modules, and a significant increase could destroy them. Therefore the power supply has to be very carefully considered.
All supply leads have a DC resistance, and in most cases this is not a significant problem. However, consider the case where the amplifier is being mounted 10m away from the operating point. Assuming that copper has a conductivity of 5e7 Siemens/meter, amd if DC supply leads of 4mm2 are used, the resistance of each lead will be 50 milli-ohms per lead, which gives a total DC resistance of 0.1 ohms. Now when the amplifier is in standby mode, the leads will drop a total of 0.3V. When the amplifier is in Tx mode, the voltage drop will be 1.8V. If this is compensated for by increasing the voltage of the power supply to give an on-load voltage of 13.8V, then the off load voltage will be 15.3V, which could shorten the life of the modules, even if little or no current is being drawn in the off-load condition. Furthermore, the drop in supply voltage as the load increases will not do anything to improve the linearity of the amplifier.
Most of the above issues relate to SSB use, but there may be a fault condition that applies even in ATV (FM) use where the amplifier is on, but no or little drive is applied. This could result in excessive voltage being applied to the modules.
The best way to overcome the problem of DC voltage drop is to use the thickest copper cable available. It may that standard twin & earrh (cooker cable) is the most cost-effective; 10mm2 cable has a resistance of 2 milliohms/metre which would equate to a voltage difference (load - no-load) of 0.6V for a 10m length, which is far more acceptable. 10mm2 cable costs around £1.50 on 50m reels, and you get a third conductor free. For a cable run of longer than 10m, it would be more acceptable to use two cables in parallel to reduce the voltage drop still further. Alternatively, 16mm2 cable could be used, but this is much more wieldly than 10mm2 cable, and as it costs twice as much, it may be more cost-effective to use 2 lengths of 10mm2.
Other options may be possible; it may be possible to use a pair of control
wires back to the PSU to regulate the voltage at the masthead. However,
great care must be taken to ensure that the feedback does not cause instability
in the control loop. Furthermore, many PSUs can only cope with a
differential drop of a few hundred mV at most.
4) Cooling.
At the time of writing, a detailed thermal analysis of the Quad has not been completed. However, for long periods of continuous transmission at full rated output, a DC axial fan of approximately 100mm x 100mm will help to keep the modules a little cooler.
It is difficult to predict how the Quad will operate when exposed to
direct sunlight with the Sun at it's maximum elevation in mid-summer.
The heatsink will act as a very good black-body absorber, but I have forgotten
the equations needed to calculate how hot it will get. It is estimated
that the temperature of the heatsink could be in the order of 40 degrees,
which would be nearly 20 degrees hotter than if used indoors at a comfortable
20 degrees ambient temperature. Therefore, either the amplifier would
have to be de-rated, by reducing the Tx power or by reducing the Tx duty
cycle, or extra forced-air cooling would be required. But extra forced-air
cooling would also need to be kept weatherproof! Also, bear in mind
that a resistor will be required in series with the 12V DC fan (or fans).
5) Remote monitoring.
It would be most useful if some form of monitoring of the performance of the Quad could be done. It is often fairly easy to check if a mast-head pre-amp is working, but not so easy to check if a mast-head PA is working.
It is assumed that the DC current can be monitored from the operating
position, which is a good indication that all is well. Ideally, monitoring
of the Tx power output should also be done. This is most easily acheived
with a directional coupler, and a dual-directional coupler can be used
to measure reverse power flow, which gives an indication that the aerial
is still connected and is radiating.
6) Drive requirements
Depending on what type of co-ax is used for the Tx feeder, there will be a certain loss of drive power. The Quad requires approximately 2 - 2.5W for full rated output. It may be necessary to use a single stage booster amplifier to achieve this, and it may also be felt that the best place for this booster is also at the mast-head. However, with a drive of 2W or more, the Quad is being driven into saturation, and a reduction in input power will result in considerably less reduction in output power.
GH Engineering will be producing a booster amplifier once the development
of the Quad has finished, but good results have been achieved with as little
as 1W drive which will give approximately 50W output.
As mentioned earlier, these notes have been produced to help those who
have shown an interest in remotely mounting the Quad amplifier, and GH
Engineering takes no responsibility for any errors or omissions.
Any comments received will be considered for inclusion in a future update
of this bulletin.
Grant Hodgson
Director - GH Engineering