Hello all ,
The more I read on this the more I become confused ! If I wish to reduce my second harmonic do I use a shorted or open 1/4 wave stub ?
Thanks all
Brad
Hi Brad
Let’s try a worked example:
If you are transmitting on 3.525MHz, any 2nd-harmonic content would fall on 7.050MHz.
Allowing for the velocity factor, we measure/cut a length of coaxial cable that is an electrical 1/4-wave on 3.525MHz. We short one end of the coax, while the other end is connected into the TX-to-antenna feeder using a Tee-connector. Exactly where can influence the result.
On 3.525MHz, our shorted-at-one-end 1/4-wave stub appears nominally open-circuit at the Tee-connector, much like there’s nothing there.
On 7.050MHz, our coax cable is an electrical 1/2-wave. As such, the short at the outer end is replicated as a short at the Tee-connector, attenuating this unwanted 2nd-harmonic.
Good-quality coax, and careful measurement, might deliver you some 25 to 30dB reduction in the 2nd-harmonic.
It’s late in the evening, but hopefully this is somewhere near correct 
73 Dave
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Hi Brad,
you can use an open quarter wave stub, this will look like a short at the connection point, OR you van use a shorted half wave stub, which again will look like a short at the connection point.
(maybe a small advantage of a shorted stub : it will not radiate any RF at the shorted end. On the other hand an open quarter wave is half the length (and weight), and is easier to tune : make it a bit longer than needed and once connected, cut it piece by piece at the open end until it is tuned where you want it)
If size matters, it can also be done with a series L/C circuit, see an example here (check the “second solution”):
Beware that connecting such a stub or L/C circuit in parallel may also influence your SWR on the transmitting frequency.
73,
Luc ON7DQ
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Brad
For hams, this is the defintive reference on stubs etc. published in 2009 by W2JVN. I have a hard copy somewhere in the house, but it seems vibroplex.com now make the PDF available.
It’s a long read and quite technical in places. Please ask should you need help.
73 Dave
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Thank you for your help everyone, I was reading an article on wide band handhelds ( Baofeng and some of the established manufacturers saying they either run pretty close to harmonic regs or breech them. As a bit of an crude experiment I tried listening on a scanner for the second harmonic and there it was relatively strong ( In the same building) So I tried walking away from it and could hear it just about up to 100 meters ish away ( other harmonics vanished after a short distance) this was at full power ( 3.8 ish watts I normally run it at 1 watt) So I then started to look for methods of reducing this second harmonic, The quarter/halfwave stub seemed like the easiest method hence how I got to this point. Am I best to keep this near as possible to the antenna ? If not which point is best ?
Thanks again all.
Brad
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Better radio 
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You’ve obviously not lived in Scotland long enough to realise that a bit of coax is cheaper than a new radio.
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Brad,
A T piece at the rig will probably be best.
73
Ron
VK3AFW
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Whilst that’s true, there are usually many other desirable features of a better radio other than just a decent suppression of the second harmonic.
I make no judgement on the radio in question here (I don’t know the brand) but in all areas of life, I’ve never regretted buying the best product I could afford where as I’ve more than once lived to regret buying something a bit sub standard.
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I fairness your probably right ! I’ve been looking for a single band handy but the prices are through the roof for some really old kit ! the original article ( which I cannot find now) did say a lot of the other more expensive modern kit is not much better. I only really use the handy from home as a base radio at 1 watt output and live out in the sticks far from other radio coms other wise I think the RX may also produce a few problems ( Having used it on a summit with lot’s of comms.
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I paid ÂŁ18 for the radio so that probably tells you everything ( although as a mobile it is better than my Yaesu FT1500 ) but it did get me back into Ham radio after 16 years away.
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I think often the first model in a manufacturer’s new product series gives you all the important features. E.g. my Yaesu FT1D 2m/70cm FM/C4FM HT with small-ish B&W LCD is still going strong after many years of SOTA use (albeit the screen steams up a bit from my breath after a while).
The later models (FT2D, FT3D, etc) added unnecessary niceties like bigger, colour screens with touch screen operation, Bluetooth, etc. More expensive and more to go wrong.
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A quick and dirty test with the vector analyzer…
2.48 m of open, no-name RG58 results in an attenuation of -28 dB at 19.7MHz and -1 dB at 9.8MHz, and a shortened factor of 0.66.
Theory confirmed. Luc rightly pointed out the attenuation of the first harmonic. Although 1 dB has no significant effect in practice.
73 Chris
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Another point to consider : a transmission line stub like the open quarter wave will suppress signals at frequency f, but also 3f, 5f, 7f (where the length is 3, 5, 7 times a quarter wave).
An L/C circuit is only resonant at, and suppresses only one frequency.
Depends on what you want to obtain of course.
Luc ON7DQ
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Useful measurements … 
For argument’s sake, I suppose better coax (RG-213, LDF4-50) could be expected deliver a somewhat sharper & deeper notch, with a little less attenuation at 9.8MHz.
But you would probably be running serious power levels in wanting to use higher-grade cable. Large HF Contest stations typically choose at least RG-213 for their stubs.
Using an LDF4-50 stub with a 2/70 handheld could well prove a little cumbersome
But lightweight RG-316 teflon coax might be useful for SOTA?
73 Dave
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Good result ! I have now tried a shorted 1/4 wave stub and the affect is quite dramatic. I used a short patch lead from the radio to connect to the T the antenna at the other end and the stub coming off at a right angle. I set the scanner at the bottom of the garden ( set at 290mhz, with the tx at 145 Mhz) and as before I received a full scale signal, I then connected the T and the stub and although the signal is still there it is much reduced and quite noisy. Not the most scientific of experiments but it has proved the point ! As I only have one patch lead at present I don’t know if this has altered the match at 145 Mhz but I’m guessing not dramatically as the repeaters in my area are still giving the same signals.
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