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I'd like to add a band-pass filter to my Baofeng UV-82 to make it comply with the spurious emissions requirements. Ideally it would be a rigid device with SMA or BNC connectors that would fit between the radio and the antenna.

I'm open to building my own, but it would have to be a foolproof design because I don't have the test equipment to verify it. Buying one is also an option as long as the price is reasonable.

You can see how bad the emissions are in this video that I found: https://youtu.be/mLjc3-gSU-M?t=6m14s When transmitting on 145 MHz, there are strong signals from 35 MHz up to 1.025 GHz.

What can you recommend? (Besides the obvious "buy a better radio" suggestion. I don't have the budget for that.)

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    $\begingroup$ Very good video. You're feeding a broadband, non-resonant DL. However, wouldn't those spurs be a lot lower using an antenna that's resonant on the band you're transmitting on? EDIT: See my answer here. $\endgroup$
    – Mike Waters
    Commented Jan 18, 2018 at 19:59
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    $\begingroup$ there are bad and good (better) HTs. see here $\endgroup$
    – Edwin van Mierlo
    Commented Jan 19, 2018 at 8:43
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    $\begingroup$ Also there are real Baofeng's and fakes... and fakes can even be bought through reputable resellers like Amazon, see here $\endgroup$
    – Edwin van Mierlo
    Commented Jan 19, 2018 at 8:47
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    $\begingroup$ Some will consider the Baofeng a "toy", and claim that they "break easily". Such claims are widespread. But one did do some testing. Testing to actually break a Baofeng, just for fun you should watch this :: youtube.com/watch?v=kZCDBsBuhmg $\endgroup$
    – Edwin van Mierlo
    Commented Jan 19, 2018 at 9:07
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    $\begingroup$ If you're seeing spurs on the transmitted signal, first check your receiver. They're probably generated in the front end of the spectrum analyser, which is wiiide open unless it's a super fancy one with a YIG front end. A simple test is to move away so the received level of the fundamental drops by 10 dB. Do the spurs drop by 10 dB, or by 30 dB? If it's the latter, then they're from inside the spectrum analyser. A better test is to use an absorptive filter or diplexer to attenuate the fundamental, before measuring the harmonics. $\endgroup$
    – tomnexus
    Commented Jan 22, 2018 at 11:11

2 Answers 2

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If you want cheap, building your own is not difficult. Here's a simple low pass pi filter:

schematic

simulate this circuit – Schematic created using CircuitLab

The values of the components should be such that they have a reactance of 50 ohms at the cutoff frequency, which should be just a little above the top of the 2-meter band, if that's your objective.

For better attenuation you can cascade two of these together. Since you'll end up in one section with two capacitors in parallel, you can replace this with a single capacitor of half the capacitance for an equivalent circuit.

Since the output power of the HT probably isn't more than 5W, you won't need unusually high-voltage capacitors. You want to keep the leads as short as possible to minimize stray inductance. I'd use large chip capacitors. At 2 meters you can probably get sufficient inductance out of air-cored coils you wind yourself from enameled wire over a pen or something for a form. Align them with their axes at right angles to minimize coupling between them.

Perhaps build it in a small cast-aluminium box or copper pipe (suggestion from comments), put a couple SMA connectors on it, and you should be golden.

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    $\begingroup$ Did you intend to have values on the schematic? I don't see any. $\endgroup$
    – Kevin Reid AG6YO ♦
    Commented Jan 18, 2018 at 22:46
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    $\begingroup$ An interesting technique is to use copper pipe and pipe caps rather than an aluminum box. You can drill out the centers of the caps and put SMA connectors through. $\endgroup$
    – user3486184
    Commented Jan 18, 2018 at 23:13
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    $\begingroup$ I replaced "are" with "should be" because that wording is what led me to think you might have forgotten the values. $\endgroup$
    – Kevin Reid AG6YO ♦
    Commented Jan 19, 2018 at 15:36
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    $\begingroup$ @mrog Wouldn't excessive voltage be just as likely to smoke the finals? Many tuners use pi filters. Here's an application node from Motorola recommending pi filters. I'm not aware of any best practice to avoid pi filters. $\endgroup$
    – Phil Frost - W8II
    Commented Jan 20, 2018 at 22:26
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    $\begingroup$ @mrog Respectfully, I can appreciate the concerns that you have expressed here. I haven't taken the time to verify the accuracy of what Phil said; however, he is one of the people here who have established a reputation here as a very technically knowledgeable person (in most cases). Just sayin', no warranties. :-) $\endgroup$
    – Mike Waters
    Commented Jan 21, 2018 at 3:04
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[This was from comments by @MarcusMüller and myself. I've moved them to this answer.]

Don't underestimate the filtering effects of an antenna. It's most important how the spurs "close" to your intended frequency are – the rest will be attenuated by the antenna (do try to at least theoretically estimate the attenuation, though!). Antennas can actually be made resonant for only a small bandwidth – maybe that's actually key to a solution here, because it's anyway likely that you'd want to use an antenna that's not a full-sized monopole :)

A recent ARRL review of a Baofeng handheld did it the way the it was done in the video the OP found --with similar results-- which was a bad example that perhaps the video's creator copied. The way to do this test would be to transmit with the stock antenna, and pick up the signal using a broadband antenna (such as a Discone) out in the open and at least a a couple of wavelengths away, and feed the received signal into his spectrum analyzer.

This is not to say that the Baofeng is a marvel of technical engineering excellence. ;-) Any transmitter should be able to use a broadband antenna --like a discone-- without transmitting those spurs, and it appears that this one cannot do that.

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    $\begingroup$ I like this answer, Mike. Thank you for mentioning this. I think about this often as I trim dipoles for the lower bands. Esp. since I also use them for SWLing, also, and due to their non-optimal cut for out-of-band signals they tend to filter out the stations I want to hear.. So am tempted to use a switch to a different set of wires for some of the SW stations I like to hunt for. $\endgroup$
    – SDsolar
    Commented Apr 21, 2018 at 0:56

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