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HF VHF receiver diplexer

LBA Technology AM antenna systems, RF
shielding, and test equipment

This is one of those posts that I started long ago and never finished. No time like the present to do the final revision and get it out there.

I have acquired one of those broad banded software defined radios, an Icom PCR-1000 to be precise, and all is well.  I am enjoying listening to various MF, HF and VHF radio stations.  However, there is a slight problem.  Very slight, almost too small to even mention, more of an inconvenience than a problem.  Still, if I am being inconvenienced, than others are too.  This issue is with the antennas.  My K9AY antenna works wonderfully from 500 KHz to 25 MHz or so.  My discone antenna works wonderfully from about 30 MHz all the way up to about 1 GHz.  In order to enjoy the full range of the receiver, I need to switch antennas.  I have a small switch on my desktop, but it seems inconvenient to reach over and switch it when going from the AM band to the FM band or something similar.  Therefore, I have decided that I need an HF/VHF receiver diplexer.  One would think that such hardware is ready made for such instances.   However, nothing I could find commercially would do the trick.

Thus, since I could not buy one, I decided to build one to add to my collection of receiver doo-dads and nick knacks.  The design is relatively easy, a back to back low pass/high pass filer system with a 50 ohm impedance throughout.  Something with a sharp cut off around 30 MHz or so:

Diplexor plot

Diplexor plot

Looks pretty good, 5th order Chebyshev filter, perhaps .1 dB ripple in the pass bands if well made.  Schematically:

HF VHF diplexor schematic diagram

HF VHF diplexor schematic diagram

Then it comes down to the building. Since this is going to be used in the UHF range, care and attention needs to be paid to the layout of the components and the design of the circuit board.  Some of those capacitance values are not standard, however, by using two capacitors in parallel, one can get pretty close.  Since this is going to be used for receiving only, I may be splitting hairs, however, I have found that well designed and built equipment is worth the extra effort.

The board layout looks like this:

HF VHF receiver diplexer board

HF VHF receiver diplexer board

I tried to keep the traces as close to 50 Ohm impedance as possible.

As one may be able to discern, C2 and C3 are in parallel to make 192 PF, C5 and C6 are in parallel to make 60 PF, and C7 and C8 are in parallel to make 163 PF.

The input and output RF connectors are whatever the builder wants to use, however, I would recommend at least BNC or type N for the VHF/UHF side.  My unit has all type BNC female connectors.   Parts list:

Nomenclature Value Mouser number Cost (USD)
C1 150 PF SMT 581-12065A151FAT2A 0.60
C2 12 PF SMT 581-12061A120JAT2A 0.25
C3 180 PF SMT 810-CGA5C4C0G2J181J 0.37
C4 68 PF SMT 77-VJ1206A680FXACBC 0.40
C5 50 PF SMT 80-C1206C500J2G 0.45
C6 10 PF SMT 80-C1206C100J2G 0.27
C7 3 PF SMT 581-12061A3R0CAT2A 0.22
C8 160 PF SMT 581-12063A161JAT2A 0.11
Case Diecast, 4.3 x 2.3” 546-1590WB 9.50

I chose a smallish, diecast aluminum case, which matches my other receiver gear.  The circuit board noted above is 2.9 x 1.7 inches, which is a little bit small.  I used 18 gauge wire between the input/output connectors and the board.

The inductors were made by hand.  I used a small screwdriver as a winding form, making the turns tightly then spreading them out to the proper distance.

Inductor chart:

Inductor Value (nH) Diameter (mm) Turns Length (mm)
L1 173 8 6 9.5
L2 468 8 10 9.8
L3 414 8 9 8.7
L4 146 8 5 7.2
L5 186 8 6 8.6

The most expensive part was the circuit board, which cost about $16.00.  The rest parts were about $18.00 including shipping.

As built photos:

HF VHF diplexor with components installed

HF VHF diplexer with components installed

HF VHF diplexer input side

HF VHF diplexer input side

HF VHF diplexer completed.

HF VHF diplexer completed.

I have installed this already and it works great. I will need to get the spectrum analyzer out and run some signals through the various ports to see the attenuation and 3 db roll off points.

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7 comments to HF VHF receiver diplexer

  • Bob M.

    Both Diamond and Comet make antennas and two-way and three-way splitters/combiners good for a couple hundred watts, although not necessarily with the ranges you need. I’ve used them with 146/445 MHz radios using a dual-band antenna but they can also split one dual-band radio to use two antennas. They contain very simple HP and LP filters and some models will support 1-30 MHz on one port. Now, they may cost more than the ~$34 that yours does, but why reinvent the wheel?

    Bob M.

  • Paul Thurst

    Bob, before I started this project, I looked at the commercially available diplexers. They all seemed to exclude a major portion of the AM broadcast band, which in my view is a downside. Additionally, their designed mostly as band pass units for specific amateur frequencies, I was looking for coverage of the entire spectrum. Besides, re-inventing the wheel can be fun at times.

  • Bob M.

    The VHF/UHF unit I had seems to just be a low-pass that cut off around 250 MHz, and a high-pass that started around 250 MHz. They passed everything within their respective ranges.

    The triplexer that I’m currently using has ports for ~50 MHz, 146 MHz, and 445 MHz. I’m guessing that the VHF port is the only one with a definite bandpass characteristic; the other two are open from almost DC to maybe 75 MHz and from about 300 MHz and higher.

    I guess you don’t really know what it passes unless you scope it out. The published specs don’t always tell the whole picture. And of course you probably couldn’t find one with a 30 MHz split like you needed.

    Bob M.

  • Rudi

    Hello Paul,

    Very nice design and construction, congrats!

    And i’m (too…) curious, what is the software
    you are using for the simulation ?

    My best 73,
    Rudi, HB9ARI

  • Paul Thurst

    Hello Rudi,

    The initial design was made by something called “diplexor” which came with the 2011 ARRL handbook. I did the coil dimensions by hand.

  • Rudi

    Thank you for the info. Paul
    73,
    Rudi

  • Russ

    I realize this is an old post, but do you have an Eagle file or something similar that you could put up for download? I also have a PCR-1000 and I stumbled across this looking for a diplexer that would do exactly what your does.

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