Adding a 160M Band pass filter to the ‘Snake’
I live pretty close to transmitters, and one of them is on 920 KHz. This puts the 2nd harmonic right in the middle of the 160M CW section at 1840 kHz. And, this thing is strong! It uses up 30 kHz on either side of 1840 kHz. I can null this thing out with the directional phase control, but it still leaves a lot to be desired and seriously hinders DX work. I don’t hear this on the ‘Windowvert’ but that is mostly due to the fact that the antennas L match and the antenna form a pretty high Q circuit and do a pretty good job of rejecting the harmonic energy.
To continue to use the ‘Snake’, and improve it’s performance, I need a passive, low insertion loss band pass filter on the input of the control box. Since there are two feed lines to this antenna, it has to be a dual filter, as well.
There is a lot of very nice free design software bundled with any of Wes Hayward’s publications, and I used one of his Chebysev filter design modules to figure out a 2 pole parallel tuned filter with Q of 200 and center frequency of 1825 kHz. The output values of the program are shown in the schematic. C1 and C3 make a 75 ohm to 500 ohm impedance converter to load the resonators, and vice versa with C2 and C4. The coils were made with 27 turns of #26 wound on a few T106-2 cores I had lying around. The -2 material is a good choice for this frequency to keep the Q high. (The higher the Q, the narrower the band pass) The cores didn’t need to be this big, as this is a receive only filter. I just happen to have these ones handy. At least I won’t have to worry about them saturating. BTW, there is a mistake on the schematic. C6 and C7 should be 357 pF.
The rest of the construction was implemented using “Sloppy dead bug low Q“ technique. I had lots of NPO ceramic caps the right value, but silver mica would be better. The ceramics lowered the performance of the thing, but, being handy, I had this thing together in an afternoon and a pair of them at that. I used a LC meter to adjust the windings on the toroids to 12 uH and then used some Q-Dope to lock them in place. That stuff is pretty handy for this sort of thing. (Liquid polystyrene). The caps were made up from various values soldered together to get the values I needed. The thing goes together in a small aluminum box with F style connectors for the RG6 I use for Beverage feed lines, and I shielded the two filters from each other with a piece of copper board to preserve the phase difference on the two feed lines.
I also added a small toggle switch to short the thing out, in case I want to use the snake antenna on other bands. Testing.. well, I am blessed with an old but working HP spectrum analyzer with a tracking generator. This gadget makes it easy to see what you have created with filters! This thing fell right off the planet at 1700 kHz and was sloping down on the other side of 1900 kHz at a less rate. Perfect!
I put the unit in place and flipped the direction phase control on (See the articles on this website on my Beverage experiences) and tuned in the 160M CW band. I use a Pro 3 with a bandscope, so it is always easy to see how much crap is pouring into the receiver! With the filter out.. there is the good old AM station blasting away and dominating the receiver pass band. I flipped the filter switch in, and……. it’s gone! Holy crap! This thing really works. I estimate the insertion loss at about ˝ dB… in other words, negligible. And what a quiet looking band now with a nice selection of CW signals.. 3 were right under all that AM garbage!
I decided to work a few stations and convince myself that this was really doing what it shows. I worked 5 Euro stations with 100 watts on the evening of Jan 5. Remarkable. Most the stations had better signal strengths on the Windovert, but with a LOT more noise too. Receiving with the snake hugely improved the signal to noise ratio and made copying very easy. What a difference.
Lots of hams living near AM transmitters have had to add a band pass filter like this for 160M and I can sure see why. It allows 160M to be used like 80 or 40M.
Here is a picture
of a single element ( There are two for my dual
feedlines.) This particular unit is one I made for Al, VO1NO for his Beverage
Here is a photo of the spectrum analyzer display. The little mark is at 1840kHz, the horizontal grid is .2MHz per division, and the vertical grid is 10 dB per division. You can see this thing falls way off when you are in the AM band area.
So, that pretty much optimizes my small urban lot 160M antennas, and I am quite satisfied with the results. It’s taken quite a bit of trial and error, measuring, modeling, thinking and building to get this all to work. And the best part, it’s been dirt cheap too. I can put out a nice low angle signal from a compromised vertical radiator, and I can receive with a steerable low noise snake antenna. I am well pleased.
If you hear VE1ZAC on 160.. give me a call.. I find every contact on this band to be memorable one.
Happy Top Banding !