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Just some brief notes regarding the installation of a 15 way D connector on the Seeed metal case in order to bring out the GPIO port for use with an antenna switch.
The ventilation slot cuts out very easily and only required a small amount of filing in order to make the connector shell fit.
First the pinout I used
End view of case
Top view of connector
Wiring to GPIO ports
Hope this is of some use to others.
Martin - G8JNJ
I've had some issues with radiated noise from a KiWi that I installed on a shared site. The main issues were that it was interfering with of VHF satellite reception and degrading the GPS performance of another system in the same room.
The KiWi was installed in the Seeed metal case, and I had fitted ferrites to the cables.
As I'm currently building up a KiWi for installation at another site, I thought that it would be a good idea to see if I could further reduce the level of emissions from a KiWi fitted in the Seeed case.
In order to do this I used an EMC H-Probe consisting of a 1"diameter screened loop connected to a spectrum analyser, in order to see what the level of emissions looked like and where they were escaping from the case.
The majority of the noise was leaking out through the holes in the ends of the case and also via the additional interconnecting cables I had fitted to bring out the GPIO ports for antenna switching.
This shows the general level of radiation from a KiWi without a case, with the Seeed metal case and with the Seeed case and additional modifications. Note that the measurements were made without the external GPIO cable connected.
The next shot shows the difference between the Seeed metal case and with the Seeed case and additional modifications in more detail.
As you can see the biggest difference is achieved by fitting the metal case, and I would definitely recommend doing this if you are still using the plastic frame supplied with the KiWi.
However there is still a fairly significant amount of radiated noise throughout the whole spectrum, especially in the VHF / UHF / Microwave range, and the only way to cure this is to close up the holes in the box.
As I didn't want to restrict airflow and ventilation I used some very fine woven brass mesh that I previously bought for antenna construction.
I added a section of mesh at each end of the box and sandwiched it between the end plates and the box sides, having first cutout suitable holes for the connectors I still wished to use.
Note that I had to solder short brass sheet straps between the Ethernet port metal socket surround and the mesh and also the -ve of the DC input connector where it enters the board (prior to the common mode choke on the board) in order to reduce the level of radiated noise from the external screened CAT5 Ethernet cable and DC power cable. There already was physical / electrical contact between the mesh and Ethernet connector, but I noticed that the screening effectiveness varied as I flexed the cable, and I didn't want to risk problems on-site at a later date if the mesh tarnished or developed an oxide layer.
Here are a couple of external views, showing the wire mesh with the case closed.
Once I had done this work I still had some noise from the GPIO port, so I added some 0.1uF decoupling capacitors and some 1/8th watt 1K Ohm resistors in series with the wires from the KiWI GPIO pins (not the 0v ground return) and the decoupled pins of the output connector. This also provided some short circuit protection, as previously suggested as best practice earlier in this thread.
After doing all of this work, the level of emissions are now minimal, and the ambient level of noise in my workshop is higher than the majority of noise produced by the KiWi, even though the close field probe is right next to the case. There is still a bit of radiation along the seams between the top and bottom case halves, but this is very low level, and if necessary can be further reduced by running self adhesive aluminium (ducting) tape over the seams.
One other interesting observation is that before fitting the mesh and additional earth bonding strap, I didn't find that using screened Ethernet cable made much difference to the level of noise radiated around 20MHz. However after performing the modification it's really noticeable that using screened cable makes a big difference.
Overall I think the additional mods are worth doing, especially if you are bringing the GPIO pins out of the box. But if you are already housing your KiWi in a diecast box with suitable cable bonding, then you are probably already experiencing most of the benefits I have observed.
Martin - G8JNJ
Some further notes for anyone who may pick up this thread at a later date and wish to copy this information.
If you wish to split the GPS signal from the antenna to feed multiple KiWI's.
Do not attempt to simply use a tee adaptor or similar to split the feed, as this will introduce unpredictable impedance excursions on the coax cables, which is turn will result in very deep 'notches' in the signal amplitude at various frequencies.
You can only use passive splitters if the antenna has sufficient gain to overcome the splitting losses and cable losses ahead of the KiWi.
2 way split = approx 3.5dB additional loss
3 way split = approx 5dB additional loss
4 way split = approx 7dB additional loss
Try to find passive splitters that only have a DC pass on one port. This will save having to fit additional external DC blocks in order to prevent one KiWi feeding DC back into another via the GPS connectors.
Make sure they are rated up to 2GHz, like this model / style
You can obtain 'F' type male to SMA male patch cables on Ebay at under $4 each. It's better to buy ones that are longer than you need, so that there is room to wrap them around ferrite rings if required. Make sure you choose the correct sex of connectors, as reverse SMA is quite common.
Hope this helps anyone else following in these footsteps.
Martin - G8JNJ
Build the LZ1AQ instead.
It's a better performer and easier to construct, plus I'm not 100% convinced about George's reverse engineered circuit, especially the transformer windings and specifically the configuration on the binocular core. Norton amplifiers bring about lots of other problems and after playing with a lot of different designs, I'm not sure they are worth the effort.
My initial IMD measurements were constrained by my test setup, with my better test rig I measure
LZ1AQ OIP2 +79dBm OIP3+36dBm
Wellgood OIP2 +57dBm OIP3+37dBm (note that the Wellgood is based on an early Wellbrook design and the new ones are better)
The overall performance of all loops, especially on the HF bands is determined by loop size and inductance and amplifier input impedance. This is why most loops 'run out of steam' above about 10MHz.
I've found that contrary to most wisdom, if you can't build a nice 'fat' low inductance loop, it would seem to be better to use an amplifier with a higher value of input impedance (tens of Ohms), in order to improve performance on the HF bands (where the loop inductive reactance dominates the feed point impedance), and compromise a bit on the LF bands, where the natural noise floor tends to be a lot higher (particularly in urban areas) and the very strong Broadcast Stations tend to be problematic anyway.
Some more info on my Active antennas web page
Here's a link I found to a lot of Clifton Labs Norton amplifier circuits (grab them while you can) that may be of interest to you and others on here (use Google translate)
Interestingly for some reason they also produced a higher gain amplifier using four Gali-74's the Z10046A, but I still haven't been able to track down a circuit of the amplifier they produced for the Pixel Loop now sold by DX Engineering (they have withdrawn all Clifton Labs on-line circuit diagrams since they bought them) as the RF-PRO-1B
Send me a private message if you wish.
Martin - G8JNJ
Adding a series tuned notch filter across the KiWi RF input will help reduce the problems on 7345KHz.
These values give the bets compromise between notch depth and attenuation on the adjacent 40m amateur band.
For this simulation I have used an inductor with a Q of 50. But if you can use a better inductor, it will provide a deeper and narrower notch.
31 turns wound on a T50-2 iron powder core would be a good starting point, but you would have to add or remove turns in order to tune the notch to the exact frequency.
It will attenuate signals on 40m to a certain extent, so the signal levels will drop slightly, however as the natural noise floor is usually the limiting factor on the lower frequency bands, I don't think it will harm the Signal to Noise ratio.
As a test you could run the WSPR decoder with and without the notch in circuit, and compare the average S/N before and after, but always compare reports of the same transmitting stations, don't compare different stations with each other.
The other interference is something local to you, but sort out the 7MHz overload before investigating the other problems.
Martin - G8JNJ