Sometimes with receiving Antennas, problems arise with interference from out of band frequencies. One of many possible solutions is an Antenna Diplexer. Here we make extensive use of our knowledge of both Low Pass Filters as well as High pass Filters to incorporate both types into one combined filter I call an Antenna Diplexer.


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I recently received an email from a site visitor who was experiencing problems with his television reception. His set up in an Australian country area, consisted of an UHF antenna and because of weak signals in the area, a mast head amplifier.

I've previously discussed some of the inherent or potential problems in the topic on Active Antennas in an earlier tutorial (November, 1999) on my old site.

I've since followed this up with a newer topic on Active Receiving Antennas where I discuss some possible solutions.

Sometimes with Antennas, problems arise with interference from out of band frequencies. One of many possible solutions is an Antenna Diplexer. Here we make extensive use of our knowledge of both Low Pass Filters as well as High pass Filters to incorporate both types into one combined filter I call an Antenna Diplexer.

How can we improve antennas by using an Antenna Diplexer?

First off Let's take a look at a rough schematic of a typical antenna diplexer without any embellishment:

This image is copyright © by Ian C. Purdie VK2TIP - a typical antenna diplexer

Figure 1 - typical antenna diplexer


Well I've depicted the Antenna Diplexer input as being 75 ohms unbalanced. It may well be a 300 ohm balanced input. This needs to be checked out and I'll deal with that later.

For the moment we will assume I'm dealing with inserting our filter into a coaxial line BUT well BEFORE we get into the mast head amplifier input terminals.

In this case, the components C1, C2, L1 and the resistor (75 ohms here) comprise the low pass filter.

What happens is we select a suitable cut off frequency, calculate our component values C1, C2, L1 and the resistor for the terminating impedance, 75 ohms here and all at that frequency.

In theory, all frequencies below our cut off will be referred to our 75 ohm resistor where they are dissipated and never get passed along to the mast head amplifier.

The next part is what we want to send along to our mast head amplifier through our antenna diplexer. Here to the right of the schematic diagram, the components C3, C4, L2 and L3 comprise the high pass filter and are all been calculated at the same cut off frequency and input impedance.

Pretty simple, pretty basic?

In the example I was asked about, the principal interferring frequency was below 100 Mhz. We could simply do our low pass filter and high pass filter calculations at that frequency. We could also do them at 200 Mhz because that's also below the UHF TV band in Australia. No matter what you do you will never get past the filter tutorials nor the calculations required.

Once you arrive at your values for the antenna diplexer you need to do some re-calculations. Why?

Why do you need to do some re-calculations for your antenna diplexer?

Well you will arrive at very odd and non-standard values of capacitance. Clever people will point out that stray capacitance will have some effect here and they would be right.

Using the low pass filter tutorial, 75 ohms impedance and a cut off frequency of 100 Mhz we should get from Fig 2, where the reactances are C1 = C2 = 75 ohms each and L1a = L2a = 75 ohms each or a total of 150 ohms for our L1.

From our reactance tutorial we know at a cut off frequency of 100 Mhz, our component values are calculated as C1 = C2 = 21.2 pF and L1 = 0.24 uH, I just rounded that up to the nearest. Don't get paranoid!

Now why not just use a 22 pF standard capacitor and leave it alone? Well in this case I would but maybe you need to work to some other frequency than 100 Mhz or even a different impedance. Often it pays to use a standard value and with a little algebra on your reactance formula to get a different cut off frequency. For example if we take a 22 pF capacitor literally (they're usually plus / minus 5% tolerance anyway) we could rework things back to 96.45 Mhz and then re-do the inductor calculations.

I'm just presenting information for people who may need to work their antenna diplexer at other frequencies, especially frequencies very much lower. At say 7.5 Mhz these comments become really significant in terms of values, strays notwithstanding.

O.K. we've got two 22 pF standard capacitors for our diplexer what about the 0.24 uH inductor? Go to the tutorial on inductance and give your calculator a work out. Here I'd use salvaged winding wire, preferably no less than 24 gge and some common bolts from the hardware supplies. Maybe I'd use say a 1/4" bolt (6.25 mm) and do some "what if" calculations on some trial runs. You can reduce or increase the number of turns and fool around with the length of your inductor, the radius being fixed. Consider other bolt diameters.

Using the high pass filter tutorial, 75 ohms impedance and again a cut off frequency of 100 Mhz you should be able to arrive at values for C3, C4 and L2, L3.

What if the Antenna Diplexer input is 300 ohms balanced?

Here it gets somewhat tricky. You need to go from your 300 ohms balanced to 75 ohms unbalanced with a balun and reverse the process at the other side.


Obviously it has to be water tight! It also needs, I think, to be RF tight as well. You can go the expensive die cast box route, make a box from scrap PCB, how about a used 35 mm film cannister shielded with craft copper foil? Use your imagination.....


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earlier tutorial on Active Receiving Antennas

Newer Active Receiving Antennas tutorial

antenna basics

low pass filters

high pass filters

narrow band filters

reactance tutorial

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