The Raging Ham Radio Debate: Where The Heck Do You Put The Band Pass Filter?
Ok, well, no one is really raging about this but me. Everyone else has just been telling me I am wrong and shrugging their shoulders when I don’t listen to them. Ever since learning of the existence of the band pass filter at my very first Field Day in 2019, I have been hooked on them. And if you think 2019 wasn’t all that long ago to be learning about filters, keep in mind I only learned of the existence of Amateur Radio is 2018. So, in 2019, all a band pass filter was to me was a way for multiple radios to coexist without interfering with one another. Once AA1F decided he wanted to play radio too, I worked on collected a filter for every ham band so that we could operate simultaneously. This expanded to group events like SOTAs and POTAs. Now I have filters for 160M through 6M, and a 2M BPF on order. I’ve used them enough to light up and have to replace the 40M one.
What exactly, then, is this great debate? Simply, it is where in your sequence of radio, antenna tuner, and antenna, do you place the band pass filter. You see, I actually emailed the manufacturer to help me figure this out. There answer was unambiguous: the BPF goes between the tuner and the antenna, with the caveat that you should tune your antenna first and then place the filter in line. Lets call this “Configuration 1”.
Now, one of my dearest ham buddies never fails to give me some indication that I am doing it wrong when I set it up this way. He knows the company told me to, and since it is my equipment he doesn’t push the issue. He just kinda shakes his head and walks away. The other important factor is I do not actually follow the manufacturer’s instructions…I tune up through the band pass filter, rather than placing the BPF in line after tuning up.
Now, just earlier tonight, a debate came up over this on QRZ, a ham radio forum. After I posted my filter set-up preference, I was told again by some knowledgeable hams that I was doing it wrong. They suggested instead switching the order to radio to band pass filter to tuner to antenna, a la “Configuration 2”.
So to lay this thing to rest once and for all, I decided to do some quick bench top testing of each configuration using my Rig Expert 600 Zoom antenna analyzer. The jist of it would be, I would use the Yaesu FT-891A as a signal generator to match the impedance of the 40M off center fed dipole with the LDG Z100 Plus autotuner in each configuration. I would tune the antenna with a 40M band pass filter in line (rather than putting in line after tuning per the manufacturer’s instructions, but instead in keeping with with the way I use the filters in practice.) I would then replace the transceiver with the antenna analyzer and sweep the VSWR across the HF bands. My expectation was to see isolation of the passed frequency confined to 40M and to have roughly identical curves between Configurations 1 and 2.
A VSWR sweep on my multiband antenna without any added hardware (besides coax) is shown below. Without any tuner or band pass filter, the OCF antenna has a SWR less than 3 on 80M, between 2 and 3 on 40M, and a minimum of 1.25 on 20M.
A 40M band pass filter was added to the antenna side and the tuner to the transceiver side (i.e., Configuration 1, see below). The antenna’s impedance was matched with the tuner and the radio was removed.
The VSWR sweep of Configuration 1 is also shown below. The 40M band pass filter certainly blocked frequencies outside of the 40M ham band. The SWR, however was surprisingly high, between 2 and 3 across the band. Again, this was after tuning up through the band pass filter. Needless to say, this actually disturbed me quite a bit…
I switched the set-up around so that I could achieve Configuration 2, with the band pass filter now sitting between the transceiver and tuner. I again matched antenna impedance, and replaced the transceiver with the analyzer for testing.
I sweeped the VSWR of Configuration 2, and found a much more satisfactory profile than that of Configuration 1. The band pass filter again excluded signals outside the 40M ham band as expected. However, in contrast to Configuration 1, Configuration 2 has a much better VSWR across the band, with most of the band under 1.5 VSWR.
So, in summary, when it comes to where to place the band pass filter, I have actually been doing it wrong. I am going to switch my radio set-up to match Configuration 2, with the band pass filter sitting between the transceiver and tuner. I do not necessarily think the manufacturer gave me bad advice, since I really am not following the advice they gave me; I do not prefer the additional steps of having to tune the antenna prior to connecting the band pass filter.
That’s that!
Yours,
KM1NDY