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Thread: Advice on directivity of wireless guitar system antenna.

  1. #1

    Default Advice on directivity of wireless guitar system antenna.

    Hi guys,

    I hope this isn't too silly a question to post to you but you do seem to be the guys who would know this. I'm a retired electronics design engineer so can easily understand technical talk and details but I am no radio engineer I'm afraid, hence the need for your advice on what must be a very basic topic in your world.

    I play guitar in a gigging band. I use a wireless system to allow me free movement and no cables to trip over or wind into. (That's important at my age!) This is a fairly basic but small, neat, modern system using 836MHz in what I believe to be a public band. My own particular frequency of use is fixed so no retuning is possible. I have experienced very occasional breakthrough from the standard culprits, taxis. This has only happened a couple of times during breaks from playing when the receiver was left on but my belt transmitter was off to save battery. It may seem highly entertaining when it does happen, particularly when the taxi controller is a bit ticked off with his drivers and his language is blue, but I'm trying to look for ways of reducing the risk of occurrence. On the other hand it did occur to me that the receiver may be more selective through simple signal strength when my transmitter is on and I am close to it so the problem may not occur and all I need to do is to turn off the receiver during breaks too. Is this likely?

    The receiver is just a neat small black box about 5" x 4" x 1.5" with an aerial which is a plain old single "twist it upright or lie it horizontal" type of about 3-4" length. It seems to me to be pretty omni-directional. I wondered if some sort of simple screen arrangement around the back and sides would help to prevent pickup from those directions and trim its polar response to accentuate me in front of it and cut down pickup from sides and back. Is this a sensible option? If not is there anything else which could be done to reduce its sensitivity to pickup from some directions?

    Thanks in advance.

  2. #2

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    I've currently got 24 radio pack systems, 12 or so handhelds, and a few guitar packs, plus 6 IEM systems.

    I'll try to do a quick run-through of wireless systems do's and don'ts. First thing - turn the transmitter on, then unmute your amp. Never deviate from this. The squelch circuit on radio receivers is very much all or nothing. Anything on the frequency, including interference will lift it and come through loud and not that clear! your transmitter will on a typical amp to guitar distance overpower the interference and all is well.

    Wireless system No.1 The most expensive radio system in the world is nearly as good as a $10 cable!

    Directional antennas
    They do exist and cost a fair amount. They're cardioid pattern. We use them with multiple channel systems. we have a rack with 6 or 8 receivers in it, and as these are diversity systems, we have two antennas feeding the receivers, and we stick them up high aiming at the stage. They work pretty well. Your idea of a screen is a bad one for a number of reasons. you've probably already discovered dead spots. Nulls where like a black hole, your RF from your pack just vanishes. What happens is your RF, quite low power radiates out all around - bar where your body or guitar gets in the way a little or a lot. It reflects off things and even if it's a diversity system with dual receivers, there will be places where the multiple paths cancel out. If you stand in one, you suddenly vanish. Move one step left and it goes back to full strength. if you create a screen, you'll make the problem worse.

    Lastly - the taxi. The taxi almost certainly isn't on your frequency, but it is probably way down in frequency compared to you, and sadly, it's first harmonic or other mathematically linked frequency is what gets through. it's also bad luck, because they're probably close, and because your system is wideband, and their is narrow band, it's just bad luck your frequency matches. However - leave your transmitter on when the amp's unmuted and all should be well.

    If the cost of batteries is a problem, go back to a cable. My biggest regular outlay is batteries. The show we're preparing for has 16 radio's. Each pack uses 2 batteries, and we put a fresh set in each show. We never risk 2 shows from one set of batteries. We never risk rechargeable - they're actually viable now, length of charge wise, but - you cannot rely on the ones in the charger being charged. sometimes, our radio person might take a set out that have only been put in an hour before - too risky. 32 batteries per show, and we have 88 shows plus rehearsals. I have 3000 AA cells sitting here ready.

    You can never predict nulls, and you can never guarantee no interference. it just doesn't work like that. For the short distance, your omni antenna is fine - just don't leave them unattended. They will splutter, and hiss - and there will be nulls. as you mention one antenna, you can improve things a lot with a more expensive system - Mine are Sennheiser G3s - which have dual receivers, and they silently switch antennas to the one that has the best signal. Better than yours, BUT they still splutter and phutt. you can also get 2.4ghz systems that are digital and pretty resilient to the problems talked about above - but these share the channel with wifi, and that's on everyone's phone and computer. My ones rarely get interfered with, but they CAUSE significant interference to wifi users who can often be heard complaining about the poor wifi in the building. that's pretty much how it is.

  3. #3

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    Many thanks for the response Paul, it's greatly appreciated. It's also good to hear from someone who knows the system intimately with its peculiar problems and workarounds.

    You mention the 10$ cable as being superior to any wireless system. With that I heartily agree. My system of choice is a long cable with head amp in the guitar plug feeding my own preamp at the pedal board. I have a couple of these I designed and built but they get only home use nowadays. (Commercial designs are poor and always a compromise and they are expensive for what they contain. There are approaches which beat them into a cocked hat on every aspect with standard components.) For one, this allows me to trim the capacitive and resistive loading for the guitar, very important to the sound character the guitar produces. I can set this up to suit my own tastes rather than have to start with the capacitance of the cable and resistance of the amp input as limiting values. Unfortunately that doesn't really fit in with day to day reality. My problem and the thing which drives me to using a wireless system is perhaps silly at first sight but I promise is a genuine issue. I play in a working pub/small club band and both of our guitarists and the keyboards all use largish gear. With a full drum setup and a bass player that takes up a lot of space. Whereas every other band seems to talk as though they play venues with stages and plenty of space, our setup is usually squashed into an area that some 3 piece bands would have trouble with. In the course of a set I'm swapping between 3 guitars, (for reasons of different tunings not for image), all on stands and once again crammed into whatever space I can scrounge around me. In a very restricted space it is surprising how much time and fuss it takes to change out a cable and make sure it is not tangled around the rest of the gear and there is always the problem of standing on and tugging the cable when playing. The wireless system on my belt is much easier and quicker to cope with.

    This also explains why the reflective dead spot issue is not too much of a problem for me as I'm pretty much static and close to the receiver all of the time. I can see that in a live theatre performance across a large stage area it would be a different kettle of fish. I haven't experienced the multipath destructive interference dropout you talked about yet but, as I'm always working very near the receiver in a small space with not much movement, I'm hoping it may not be an issue for my needs. If it does occur I will have to note it and see how I can work around. It is easy to check out the area I would have in the setup before a gig. I would imagine simply moving the receiver would alter the situation significantly to make it workable.

    I am in the process of making up a small belt battery pack based on Li-ion rechargeables. This will feed the transmitter via a spare battery cover over a dummy battery. I have a battery carrier in a belt pouch for 4 batteries. I will use them in pairs to give about 7.2V->6.0V and regulate the voltage down to 1.5V. With a switch on there to swap over to the other pair on the fly, it should give me more than enough for the longest of our gigs. I could do this with a single Li-ion and modern low dropout regulator chips but the higher voltage than necessary makes it easy to add battery condition warnings too and is a little more forgiving on discharge depth. Swapping to a charged pair could even be made automatic though I'm a fan of the KISS approach, Keep It Simple Stupid! I already use a compact 18V cordless drill battery (with a correctly fitting manufacturers head cap) to power my pedal board power supply without issues so recharging is already a part of the ritual for every gig.

    As I mentioned, this problem has only ever occurred during a break with the transmitter off. It's good to hear you confirm that there is a degree of selectivity based on signal strength. I'll soldier on and see if it ever becomes a problem when playing live. Many thanks for your insight and sharing of your experience.

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