Adding a DC input to a UHF masthead amplifier (and reverse engineering it in the process)

Recently I was looking around for a one input, one output 75Ω UHF amplifier only find such a thing is either extremely rare or non existent. A masthead amplifier fits the bill, but won’t ever have a dedicated DC power input because they’re fed from an indoor injector unit, which I just don’t need in this instance, because it’s all indoors.

Normally a distribution amplifier is used in this scenario, but, I don’t want a distribution amplifier. It’s sub optimal and overkill for what I need.

So I went down to the local DIY shop and bought the cheapest masthead amplifier they had with the intention of bodging a DC input into it. This ended up being the Maxview MHA26U. Can’t be too hard right?

I whipped off the cover, and had a look see what’s inside:

It’s fairly easy to see what the solution is: Just de-solder that inductor from the output, and onto a newly added DC input.

Like this:

Voila. Note that I ended up using a different inductor in the end, because I broke the one it came with. What I’ve done here is move the DC power path from the output, to the DC jack I’ve just added. Easy.

Not so fast…

Before I go pressing it into service, there is a claim on the box, “NOW WITH 4G FILTER” which I’m interested to investigate. There is bupkis in terms of technical information on the box or in the manual about what kind of filtering this actually refers to.

In the UK 4G is either in the 2.6 GHz (Urban) or 800 MHz (Rural, and maybe urban in some cases) ranges, the ones UK telcos nearly bankrupted themselves bidding for. For this to have “4G Filtering” I’d expect some kind of bandstop filter in this range.

Filtering 2.6 GHz is largely pointless as television antenna cables and components are very lossy at these high frequencies. The filtering is already included in the (lack of) price.

So really we’re just looking at the 800 MHz range.

There is quite a bit of filtering at the input. Is this our mystical 4G filter?

There are two filters here:

  • Bandstop filter – just a simple LC arrangement.
  • Lowpass filter – a slightly more elaborate 11th Order Inverse Chebyshev design

I went to the trouble of isolating the two to see if either of them are 4G related. First up, it’s the bandstop filter:

Nope. This isn’t 4G related. It’s most effective between 240-365MHz, with at least 20dB of attenuation below that, so this is more of a VHF blocking filter than 4G. Actually a good thing to see in here, because if there are VHF signals present, they’re likely to be quite strong, potentially saturating the amplifier which we definitely don’t want. Most importantly however there are no television signals in the VHF range in the UK anymore.

Next up: The Chebyshev low pass filter:

Once again, not really 4G related, but kind of. It tails off sharply at 800MHz so you could argue that this blocks 4G signals, it will but that’s not really what it’s there for. There is no point including anything above this frequency as it will not benefit, and in some cases may impair digital television reception. 2G signals, typically 900 MHz, the ones that make an annoying buzzing noise in your speakers are more of a threat than 4G .

Putting both of the filters together:

In this sweep the amplifier is included and set to 10dB gain

It’s a nice UHF bandpass filter with a clean response from 425-800 MHz. I think the 4G filtering claim is a bit bullshitty.

What I measured above agrees fairly closely with the frequency range stated in the supplied manual. Not quite, but close enough.

I found another version of the MHA26U’s manual on the internet, and interestingly a different frequency range is specified. This reveals what the “4G filter” really means. The MHA26U always had a good input filter, however when 800 MHz portion of the UHF television spectrum was removed and re-allocated to 4G, this product was revised accordingly.

This does highlight an interesting point about these products in general. At the time of writing Ofcom is busy clearing the 700 MHz range for use with mobile devices, most probably this space will be sold off for 5G use, so we can expect there will be yet another version of this product with likely a 470-700 MHz (ish) range labelled with “5G filter”. This is also either underway or complete in other countries too.

The conclusion here is that a reasonably important technical detail has been buried under a vague marketing claim. If installing a masthead amplifier (or any other similar product) check that what you’re buying is up-to-date for the spectrum in use at the time.

Test setup

These tests were performed by a 50Ω spectrum analyser with tracking generator using two matching pads, and I’ve got an attenuator/limiter in there too just in case. A setup like this of course has its own complex characteristics, these were factored out before testing using the F to F female connector pictured.

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