Here are some measurements of 3 filters off the low pass filter board I designed and built. First harmonic of each band is at least -50db down. The 4th band which in my case was for 15m, was well out and I need to look at it to work out why, cap values is going to be my initial guess LOL. I built this thing and have never used it LOL. Though I got some new boards coming from JLCpcb this week and i hope to have a transmitter happening that I will use in combination with the HackRF One as a receiver.
I had reason last night to use the Nano VNA for something and while i was at it, i grabbed down an Xtal filter i had built ages ago and put it on for measuring. If i recall right, and I would have to go and look in old blog posts, when I measured this using the Bode Plotter in the Red Pitaya it looked alot like Bart Simpson and had 5db of ripple in the pass band and very lossy. Well, better tool, better measurements and this does not look all that bad to me. The ripple is acceptable and actual loss in the filter is around 3db which is more than fine. Its not quite narrow enough for a CW filter, but its good enough to get started with.
There was a post over on EEVblog that got me thinking. What do you do when you are unable to, for various reasons not be able to procure iron core toriods. Well, obviously air core inductors are a thing and highly practical for the low values of inductance used at VHF and UHF. But, what about at HF?
Well it turns out that as long as you do not mind a little bit of size to the inductors, they are still a vary practical option. So what i did was grab out some 1mm enamelled wire and wind 9 turns on a bit if 22mm OD pipe i had laying about to see what sort if inductance such a coil would create. Turns out about 2uH. That is highly usable value in low pass and band pass filters in the lower HF range. And with some thought on the actual construction and mechanics of the coil, very usable inductors could be made this way if you are unable to buy iron core toriods for whatever reason.
Anyway, that is some food for thought. There is nothing magical about iron core toriods, air cores will work just as well and should have a reasonable Q equal to or better than a toriod. So if you are stuck, there is a solution.
Today I got all excited and thought about seeing what kind of range of usable range i could muster by connecting a small amp to the DMR hotspot and connecting it to the outdoors antenna. I was putting about 0.5w out into a slim jim about 3m off the ground. I could still hear the kerchunkers about 1km away. Nothing stellar but you really could not expect to much giving the setup.
So after that I got all excited and connected the Hackrf to the same little amp and again to the outdoor antenna and fired it up in FM CW beacon mode and got an ultimate range of about 3Km. Again nothing stellar but considering everything, not to bad either.
So the moral of the story here is I really need to get the antenna much higher into the air if I want to be able to turn this into something a little more usable without resorting to much higher power.
I got excited last night and grabbed some of the piles of junk out of the piles of junk draw to give a go and see if i could do anything with them. First off the ranks was the Pixie, its god awful but works. I did not manage a contact with it last night but was spotted by the guy up the road on RBN.
This morning I sent a mate in Sydney a message and got him to listen out for me and after he opened his filters up to find where I was, we made the exchange. Brisbane to Sydney with a Pixie, 750km or there abouts, so that was SUCCESS.
Next I busted out the Frog Sounds, I dont know why but its not working on receive, but it transmits just fine and is making 1/2 to 1 w. So i called CQ a few times just to see if I could make some RBN spots and low and behold i got a spot in New Zealand. 2600km. So that was a success also.