RLC Low Pass

RLC Low Pass Circuit

From Wikipedia:

low-pass filter (LPF) is a filter that passes signals with a frequency lower than a certain cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter design.

I’ve incorporated the circuit into my schematic:

This is going to be very easy to install and wire up – I can’t wait to test it out! I’m finally going to have an Inverted L tuned for 160, 80 and 30M without the need to use the AT2K – and where just the SPE ATU will work. For other bands – if I need to use the Inverted L as a backup, I would use the AT2K as well.



80M LC Circuit 

It was easy to get the LC circuit to work, as I have two excellent components. The roller inductor is a Palstar inductor and the variable capacitor is from an Ameritron ATR-30 where I had to constantly fix the edge wound roller inductor, so I junked it and am using it’s parts.

They both fit at the top of the box and will easily replace the UNUN that didn’t quite cut the mustard. If I want, I can even use the URAT make the circuit remote tuneable – so  this project has an evolution that tracks my learning.

And I can tell you that I have really stepped up a couple notches by doing the URAT and now this. Its been the most fun I’ve had in ham radio, (and I did worry if there would be life after DXCC).

Well, there is!

Calculating the 80M LC Circuit

I was able to find an online calculator that will give me the circuit components for an 80M LC tank circuit:


I happen to know that my Palstar roller inductor goes up to at least 20 uh, and while my giant Palstar variable capacitor goes up to 450 pf at 4.5 kv, I’d like to try to use a smaller E.F Johnson variable capacitor that is also high voltage but has a smaller footprint. Its capacity is 20 – 251 pf.

When I plug in values near the middle of each component, I get resonance between the 75 and 80M band. This means I should be able to get a much better match at the antenna and then I will be able to take the AT2K out of the circuit as 160, 80 and 30 will all be within range of the SPE amplifiers ATU range. I will probably keep the AT2K in line – in case I have to use the Inverted L on another band that will require a tuner, but keep it switched direct bypassing the actual tuner.

So – this relay switching network has proven to be an invaluable tool and “platform” for testing and playing with antennas and getting the match just right. There is nothing as good as simply making the antenna resonant, and while that is easy to do on a mono-band antenna, its really tricky when you try to multi-band an antenna (at least beyond two “sympathetic” bands like 160 and 30M).

Multiband Antenna Switching

When I first started on this project I think my goal was to “magically” handle SWR on several bands and my area of focus was on using an UNUN or Current Choke to do so. They did do what they were supposed to – to a point, but there was no “magic” there. They did bring the match “close” to the range of the SPE Expert 1.3 amps internal ATU, but not quite close enough.

Knowing that the antenna was a little short on 160M, I added 10′ of wire which brought the resonance on 160M down into the CW portion of the band nd this is with the 1:1 current choke “Position 3” switched in:

The SPE ATU now handles 160M without the need for the AT2K!

and with the 1:2 UNUN in position 2:

30M also no longer needs the AT2K tuner, the internal SPE ATU is fine.

80M in Position 2 is improved, but not good enough. HOWEVER, because Position 1 is a total bust, I will replace Position 1’s 4:1 UNUN with a vacuum variable cap or other tuned circuit to give me a good match on 80M.

In retrospect, I could have probably just hung three loading wires off the top of the 41′ vertical and trimmed them for 160, 80 and 30M and achieved something similar – at least on 160 and 30, but I suspect 80 and 75M being a spread of 500 khz requires some kind of switched or variable tuned circuit.

I’m going to guess that 80 and 75M will need some variable capacitance. I might first try to just add a variable capacitor replacing the 4:1 UNUN, then try a tuned tank circuit. What’s really nice is – there is enough room out in the box, and I have plenty of control wires left if I care to remotely tune this tank circuit as I did with the URAT.

Lets look at what adding 10′ of wire did overall:

Position 1 – add 10

Position 2 – add 10

Position 3 – add 10

I think using Position 1 for 80M will be a winner, and I will be able to obviate the need for the Palstar AT2K tuner.

Mission Accomplished!

The switched transformer Inverted L matching circuit works as designed. The biggest take away is that now I can run full power on 80M, which was the biggest challenge. This morning I woke up, came out to the shack and did one last test and found that the 160M position was not isolated when I was switched in position 1 and 2 – so I had to add another relay:

I know it has something to do with the fact that I changed the 160M position (which only goes through the 1:1 current choke) to be a switched on position instead of a default position. The additional relay ensures that all three positions are isolated – that there is no way to transmit back into another current choke or UNUN. I’m still scratching my head – but I probably need to create a switching “truth table” that has the NC, NO and voltage ON positions – then I might see what I experienced with the DVM. It seems redundant to have that new relay . . .

The biggest deal is 80M, and the fact that now the tuner sees what makes it happy range wise – so there is no folding back over 300 – 400 watts, and I can run full power. When I look at the Rig Experts AA-30 plots I don’t see why this is the case, but on the air, everything is great.

So while it has all worked out – there are two “mysteries” to solve with data. And this will be fun, but for now I can kick back and enjoy success with something I designed from scratch and built that helps me in a very practical way.

The UNUN’s “almost” obviated the need for the AT2K, but the SPE Expert 1.3K ATU is just “OK” – it really can’t match a 3:1 – its more like 2:1 which is the biggest disappointment with this amplifier – the Elecraft KAT-500 is WAY better. I would not recommend the SPE ATU at all.

But all in all I’m happy with this latest project – I’ve learned a lot and really like a switched circuit way better than one that uses a Stepper motor as the URAT project did. I’ll be pursuing high power switched circuits from now on.

I have two 160M 1/4 wl stubs at 75 ohms and will play with feeding this contraption with those stubs. I also have a massive coil that Ed, AG6CX gave me – and will see if maybe a switched hairpin is another option.

Thankfully, while DXCC runs out of gas after so many years, experimenting and trying new things seems to be an endless pursuit – hence why this hobby is as much a journey and expedition as VK0EK was!

Solar Cycle 24, 3Y0Z, and the Downward Trend

Wow – it seems like we are closer to the bottom of the cycle than I would have expected. Look at the wild swings we have had in the last few months. Luckily, even with zero sunspots, 40M has been a beacon to ZS and V51 on the evening short path (only West Coast path for 3Y0Z) and the morning long path.

Can you believe it? 3Y0Z should be on the air in 6 weeks. I guess I will use the 11 days I have off between Christmas and New Years to get everything ready. I’ll use my “Air Boss” pneumatic antenna launcher to get that 17M EDZ up in the trees – and pray the squirrels don’t eat the rope before or during 3Y0Z.

I think 3Y0Z will be the last real hurrah for Cycle 24, at least on the high bands. There will be a time when even 20M will grow dim and I will re-purpose my 2 element 20M yagi to be a 30M Moxon – all aluminum since the dimensions width wise are about the same – all I need to do is get some really thin aluminum from DX Engineering and make the two side pieces.

This week we had a glimpse of some great Top Band conditions. JA, BA, HS, VK and JD1 were all quite good. So, as the high bands fade, the low bands will become good. Top Band for the most part has been disappointing this Fall – but I do remember how things can get better in the first 3 or 4 months of the year.

I think DXpeditions will start advertising what great low band antennas they have – and I think this will get more people on the low bands – if people have the room (and patience) to get something half way decent on 160 and 80. 40 and 30M are much easier to get a decent antenna in – phased verticals offer best bang for the buck where maintenance is very easy. Its amazing how much time I’ve spent on “simple” vertical low band antennas!

Gee Santa Claus – how about 8 Top Band QSO’s and ONE 3Y0Z QSL card? That would be swell!

The Greatest Invention: The Relay or Switch (?)

from “Lightning Man, the accursed life of Samuel F.B. Morse” by Kenneth Silverman

While working on my Inverted L switched transformer circuit:

And simultaneously reading how Alfred Vail helped Morse create a way to make the Telegraph viable, it occurred to me that without a relay – an electromagnetic switch – Telegraphy would not have taken off. The relay is what let you daisy chain circuits together that could power each segment with batteries up to a certain distance, and that the distance was greatly increased because the relay was the link between chained circuits.

One thing leads to another, and my circuit has logic to it – and so I started remembering that computers are nothing but massive chained circuits that is a set of switches. The switches being tiny inside semiconductors, and programmed just as I have programmed my little antenna matching circuit to behave based on chaining relays:

Some may say the battery – DC power – or AC electricity was the greatest invention, but without a switch, these would not be practical. The relay and Telegraph are certainly two of the most important inventions in human kind..

I grew up near the very historic Morristown, NJ and Speedwell Village, home of the Speedwell Ironworks where Morse’s first relay circuits were perfected by his assistant Alfred Vail – and with a design based on Joseph Henry’s work.

I am almost sure we went to Speedwell Village in Morristown on a field trip – we went to Morristown a lot and also went to Menlo Park, NJ where Edison had his lab. In fact, the first three semesters of College were spent at County College of Morris – also very close by. Synergy? There were so many great inventions from NJ – Bell Labs and AT&T, and the work by Major Armstrong in Alpine, NJ, etc, etc

Living history, when combined with building stuff in the Ham Radio hobby has a way of giving some very interesting perspective on the achievements and evolution of technology.