HF/VHF Diplexer

One band I've been wanting to activate is 6 Meters, or the magic band as it's sometimes referred to. Probably like most amateurs, I own a do-it-all rig - the IC-7000. My first challenge was I didn't want to use a coax switch to flip between HF and VHF. I know there would come a time I'd forget to flip the switch and transmit into the wrong antenna. The other requirement I had was not to have another coax run out of the shack and up to the roof. What this calls for is a diplexer. A small passive device that will automatically "switch" between the HF antenna and the VHF antenna. It's really just two filters, one high-pass, one low-pass with the input being in-between the filters. HF goes out the low-pass "side" or port; VHF goes out the high-pass port. After looking at the offerings from various amateur radio manufacturers, I just couldn't spend $70 USD or more for essentially a box, 3 SO-239 connectors, and some capacitors and inductors. This is what I came up with:

Using Elsie electrical filter designer from Tonne Software, I designed the following filter. Elsie will allow the design of numerous filters; some of them being low-pass and high-pass. What's important in the design of a diplexer is the frequency that anything lower in than this frequency is considered HF and anything higher is considered VHF. This is called the crossover frequency. Calculating this frequency is rather straight forward; take the geometric mean of the highest HF frequency and the lowest VHF frequency. The highest HF band is 10M, with 29.7 MHz band edge, and 6M is 54 MHz lowest band edge. I bumped the 10M frequency to 30 MHz to keep the math easier.

My design equation:

Solving that equation equals a crossover frequency of 40.25 MHz. In a nutshell, this means any frequency <40.25 MHz is considered HF and any frequency >40.25 MHz is considered VHF. Here's what Elsie calculates for inductors and capacitors for this diplexer:

<<put scan of graph>>

Time to hit the bench. To get the best performance from this diplexer, I wound the coils out of 16ga enameled solid copper wire and used 500Vdc silver mica caps. An etched circuit board is not needed for this project, I used a couple of terminal strips, and for the case, I use a common plastic electrical box from Cantex. I had all the parts except the case, which cost me $6 at my local Home Cheapo hardware store. Diplexers are not that complex to build but a few things need to be kept in mind during construction.

  • Keep leads as short as possible.
  • Use high quality SO-239 connectors.
  • Try to keep the coils at 90 degrees to each other to limit coupling.
  • Have a solid ground throughout.

Building this up took about an hour. I used a blank, single sided PCB, and two recycled terminal strips. Wound all the coils and measured them on my AADE L/C Meter IIB to verify their values. For good measure I also verified the mica caps as well. Notice how the coils are as close to 90 degrees apart from each other so their magnetic fields won't couple into surrounding inductors. To verify the crossover frequency and overall SWR, I used my MFJ antenna analyzer and made up two dummy loads out of carbon resistors. One dummy load was 50 ohms and the other was 75 ohms. With this dummy load arrangement, when the SWR started climbing higher than 1:1, that was my crossover frequency. By the time the SWR reached 1.5:1, I knew the RF signal out of the analyzer was going out the port that had the 75 ohm dummy load attached.

I used a step drill bit to drill the holes for the SO-239 connectors. The sides of the box were very thick so the next sized step on the drill made nice recesses. This ended up working out to my advantage.

Here are the SO-239 connectors installed and sealed with silicon sealant. The recesses made by the step drill made it very nice to seal the enclosure. The enclosure comes with a lid and a gasket to help keep the elements out.

Here's the diplexer installed in the enclosure. All of the tabs on the SO-239s are soldered directly to the PCB, and all components that are connected to ground are also soldered to the PCB. I wasn't shy with the solder either, I gave the grounds a good blob.

Here it is out in the wild, all sealed up to keep mother nature out of the sensitive parts. The boots are made from an old bike tube, I read about this in Hints & Kinks in QST. I made slight modifications to use them against the side of a case. I cut the tube about 3" long, then I folded one end inside the tube to create a fat end. After placing the tube over the connector, I pushed the fat end against the case, then tied it off with a tie wrap. This makes a nice seal against the case. It's not waterproof per se, but it should handle rain just fine.

How does it work? Most excellent! The crossover frequency was 42 MHz before the SWR started rising. Not too bad, what's 250 KHz among friends anyway? Scrounging up parts to build a diplexer can save a bunch of money. It would even be possible to build a HF/VHF/UHF triplexer, or a 2/440 diplexer as well using the same process, but with a different frequencies used for the crossover frequency.