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- 50 to 250MHz span YU1LM filter before mods
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- 100 to 200MHz span, after mods described below, markers at 138, 145.9 and 153MHz
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- Experimental 2m filter/preamp
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- Experimental 2m filter/preamp
(*** see updates here and here)
As you are aware one of the key features of the FCD is its wide frequency range of operation. While this is great, the compromise with wide band receivers are that they can suffer from desense from out of band signals.
Although the FCD has programmable and fixed front end filtering, at VHF the filters are quite wide and if you’re in a strong signal area, popping in a low loss front end filter will help here.
I’ve built three 2m band pass filters based on the design here.
What I’ve discovered is that you really do need to use the PCB layout with the through hole links, and you must use screening around the unit either with PCB material or for example some brass shim. Without these considerations I found the filter lossy and difficult to tune.
In addition, I found the inductors as specified made the filter LF at 116MHz, so a fair bit of stretching was needed. Luckily I have a VNA in the kitchen lab to do this although you could have a go yourself, tuning for maximum signal on a beacon.
I managed 1.2dB loss with about two or three minutes of coil massage. Probably I could tweak it a bit more given time.
The pictures show the filter and its response. The span is from 50MHz to 250MHz. Thus, in the UK, it might not be the best if you are suffering close in pager interference around 138 or 153MHz where notch filters will provide better immunity.
Howard
Hi Howard I have a small number of punched “filter boxes” that might be usable for this purpose. They are punched for BNC but I also have some unpunched ones. I could probably get some punched for SMA but I guess most people could drill a couple of holes and save 30p :-))
The is a picture part way down the web page quoted above.
Great Work….Thanks
Alan G3NYK
Hi Howard,
I was thinking to use some surplus filters ; I have them from long time and I have used them with elder tech rxs in the past. One is a very small helicoidal filter, 5 cells, thae was used in Autophone mobile riogs for police etc. I do not know its insertion loss, but it’s very good to use it for listening if you have strong signals in the band.
For weak signals usages, I used these times a surplus silvered medium size single cavity filter, very sharp, on the rx cable before the antenna in of the RX but after the power RF switches & eme pre that are on the antenna itself.
When I’ll get the FCD, I’ll notify you; hope that the address error reported by paypal will not affect the arrival time , should not .
73 de Augusto
Not sure as to what type of front end filtering
Is required here…
Check here under Amateur radio. Myself use the
Satellite units which work very well.
http://www.dci.ca
Ern
Hello Ern.
Thank you for your note. Absolutely, these work, I have a set myself for 144 and 435MHz. But they are twice as expensive and about 100 times the volume of the FCD though!
Howard
Update: I spent some time understanding this filter a little more in an effort to narrow the filter’s passband.
I changed the inductors to 97, 101 and 97nH respectively. The 10pF caps are changed to 8.1pF. The 3.3pF caps are changed to 1.8pF. All caps were 0603 size.
I found that if you wind the 1mm diameter copper wire coils on a 5mm drill bit as a former you end up with an outside diameter of 7mm as it springs out a bit. This works with the above inductor values, but to tune I had to pull them out to about 11 or 12mm.
This gives a slighty increased 1.4dB insertion loss, but with the benefit of being 10dB down at 138MHz and 153MHz. Not as much as I’d like, but a step in the right direction. The circuit simulation suggested 16 to 17dB down, I guess I need to figure out some higher Q coils.
Howard
I have been experimenting with using preamps. Although we don’t really need much gain, it seems that one means of getting a narrow bandpass filter without excessive losses might be to use an active method. I’ve been experimenting with a BF998 design, it has 138MHz at -30dB and 153MHz at -48dB. Elated as I was, I then discovered that the noise figure was dire, so I need to do a bit more fiddling. See pics 4 & 5 in the gallery at the top.
By the way, the next FCD PCB spin will probably have software switchable 5V phantom power for such a preamp.
Howard
Receiving my funcube , After purchasing a sma connector to 239 I found that as shipped (not as pictured ) Wher I live the smb and smc. dont know which is the new connector is next to inposible to get . any have any suggestions? is it a c or b?
Jimmie
I can assure you that it is an SMA. I test every FCD it with the same setup and that has an SMA plug on the end of it.
Are you sure your adapter isn’t an RP (reverse polarity) SMA?
Howard
As I stated in my email , to Howard , There was a dead short between the floor and the key board. I accepteed Radioshack ‘s markings a being accurate . The connector is a sma ,not smb or smc as stated . by the way the sma to 239 cables can be purchased on ebay for $3.00 not $19 as radioshack charges . You still have to buy a coupling. sorry jim