rc-j6

have a Futaba T7CHP Radio. It came with frequency 40.830 (channel called 83 here)

Ive used other frequencies eg 40.810 (channel 81)

Can I use 40.835 or 40.815.

Does it have to be retuned to use the 40.835 40.815.

The guy in the model shop said just buy a new matced pair.

Dont worry about the legal freqs used here as they are not published. Its a developing nation.
also by using the .835 .815 type frequencies are they likely to be used by other industries, or would it be too close and they wouldnt use them.
The normal RC frequencies her are 40.830 40.810 etc and 72mhz


Before any Aussies flame me I am not living or flying in Aus

redfox435cat

then where are you? In a developing nation you can use whatever you want. I would recommend getting a spectrum analyzer to make sure what ever frequency your wanting to use is not being used. This one doesn't do 40 so I guess it wouldn't work but this is what you'd look for. use[link=http://www.aerospectra.com/brochure.pdf#search='rc%2072%20mhz%20channel%20ran ge']analyser[/link]
You can't just change the Tx frequency, well you can but your taking risk in that you system won't be tuned for the channel your using and your range will be greatly reduced. send the unit to a tech or futaba and have it tuned to whatever channel you want
In the US
channel A1-A6 is 27MHz all use

channels 00-09 50 MHz HAM band for general aircraft use

channels 11-60 72 MHz general RC aircraft use

channels 61-90 on 75MHz general RC surface

daveosoar

Hi Red Fox,
I'm puzzled by your remarks about changing Tx Xtals.
Here in the UK, where we fly on 35Mh, it is usual for most people to turn up at the site with a bunch of both Tx and Rx Xtals so that if the frequency one has in the Tx is in use it is a matter of seconds to change it to one not in use, doing the same with the Rx to match the number. Never heard of a range problem even with 1/4 scale gliders at the limit of visibility!
Tell me more.
David.

daveosoar

Sorry, should add the bands available are:
27Mhz - All uses.
40Mhz - Surface craft only(cars/boats)
35Mhz - Aircraft only. Most organized sites run gliders on even fqs., power on odd.
David

redfox435cat

I didn't say it wouldn't work. I meant you shouldn't do it, most of the time you probably wouldn't notice, esesially with gliders were there is no engine generating any RF noise. so people think it's OK, just because people do this doesn't make it OK, again your not in a place that regulates it so it doesn't matter. The RF harmonic in the Tx circuit are not totally dependent on the Tx crystal, the Cap/inductor resistor ladder circuits as well as oscillators and preamp post amps and the length of the anttena all have some play in it as well. I'm sure someone will buzz in on full radio 101 how it works presentation, the point is it's a tuned system and changing the crystal disrupts that tuning. Todays radio have more and more IC's and SMT components so it does lessen amount of shift off frequency and changing within 3-4 channels still not recommended probably won't be noticed. Definitely don't change between full bands

rc-j6

Thanks for the comments.

I do think its strange though.
I have been changing my TX crystal between 40.830 and 40.810 depending on what model I wish to fly.

If the TX needed retuning by a qualified tech at every crystal change (within the same band range) why is the Crystal easily accesable on the outside of the TX. Shouldnt it be inside so only a Qualified Tech would likely change it and AT THE SAME TIME retune the TX for the new freq.

I was not quite accurate with my comment on been in an undevelpoed nation. That was just to indicate that changing freqs did not have any legal implications.
It is more advanced than that and there are model shops here. They use 40Mhz TX and 72Mhz, possibly 35Mhz.

The question is,-- Does the TX need to be retuned moving from 40.810, 820,830 etc to 40.815,825,835.
I bought a channel 83 RX crystal from Germany, however thats a 40.835 not 40.830. As they are much cheaper there than here I was trying to ascertain whether I could now buy a 40.835 TX and RX crystal from there and use it safely.

redfox435cat

I don't know the purpose of them being so accessible. couple though would be the radios are sold worldwide so again areas were the radios are unregulated people can do what they wish with them also it's true verification of what channel your on by physically checking the crystal.

your frequency's are close enough were they probably wouldn't harm anything. If you went from 40mhz to 72 mhz I'd worry about it.

Here in the US is illegal to change the Tx frequency, There are two exception. One is Tx modules. which is still basically it's own Tx with all the tuning circuits in a changeable module and the PLL transmitters that re-tune themselves to each frequency without crystals this rule exist for a couple reasons. The radios are for public RC aircraft use only. by having a transmitter set to a specified frequency and is not to be altered by unqualified people is how the public is allowed to use this type of equipment without a license. As with any law that exist here is was created because someone got sued. Probably didn't make sense but hay

PlaneHeli

It sounds like the US is stricter than other countries.

So to anyone in Europe, Asia, Australia, and New Zealand. -- What is your countries policy on changing the TX crystal (staying within you band though).

It seems most countries dont require a license for a RC TX nowdays which is great. Seems a bit pointless requireing one if the TX is designed to only TX on approved freqs.

daveosoar

Hi All,
Very interesting! The UK FQ bands are as I described above. When buying a new radio outfit at the local model store its usual to be asked which Fq. you prefer. Having made your choice a pair of Xtals are then produced for you to fit, plus at extra cost, any further pairs one wants.
Its odd how different countries have different standards. In the UK you have to pass an exam and buy a licence for any form of voice transmission except cell phones or 'walkie-talkie' units with a range of less than 3Km. The 25W VHF on my boat costs £75.00 in licence fees, must display the licence, and be operated only by a qualified person. The boat does NOT have to be registered and anyone with enough money can buy a 75' Powerboat, jump in and go!
Crazy???
David.

PlaneHeli

So in the UK you can change Xtals.

I bought a 40.835 Xtal in Germany, so I believe you can change Xtals there ok.

What about other countries in Europe, Asia, Australia and New Zealand.

bhole74

Just don't move to California, changing crystals there causes cancer!

daveosoar

My daughter lives in California. On my visits i've noted that everything causes cancer - except gasoline fumes.

Vogan

There are no rules in New Zealand about changing xtal's. In your example a change of 5kHz (probably even 10 or 20kHz) won't make that much of a difference in the radio system performance. However going from one end of the band to the other will probably cause issues. This is because the RF output stages are tuned to a particular frequency (ie the one that it is sold with) in the factory.

NemisusDM

My Feedback: (3)

Hello,
If the crystal is compatible with your TX'er, RX'er then it will make no difference going up or down 5khz. The circuits are so wide that it will never notice the difference.
So the reason for the crystal change is to put all your RC stuff on one channel and the German crystals on odd frequencies are just a cheap alternative?

rc-j6

Thanks Nemisus,

Thats correct.

I bought a channel 83 xtal in Germany, but when I got it home realised it was 40.835 (channel 83 in Germany). not 40.830 (channel 83 here)

I cannot buy reciever TX and RX xtals separately here so thought I would buy again in Germany.

I have already bought a pair of channel 81 40.810 xtals here and they work fine. Should have just bought another pair of channel 83. But thought it would be good to have an alternate freq in case mine was in use. Not applicable here I found out later, as only a couple of modellers turn up to the flying site and theyre on 72Mhz.

Will save me changing xtals every flight as I alternate flights between heli and plane, often 3 flights each aircraft a day.

Just wasnt sure if the TX required tuning for using 40.835, 40.815 compared to 40.830, 40.810.

I will not be able to use these xtals, or possibly even my TX, RX when I return to Australia as they are not approved freqs.

Just a bit annoyed for buying the wrong xtal in Germany (my own fault) and the fact that xtals cost 4 times the price here than in Germany, and they dont sell RX xtals separately here. When LHS put huge mark ups due having a monopoly in a country, I will buy outside if I can.

I also didnt realise there was a limitation on using freqs within your own band. I always thought I could use any freq. as in 40.660 to 40.850 with the same TX.

PlaneHeli

daveosaur,

Yes interesting. You say the model shop asks you what freq you want when you buy your Tx. yet if it was only tuned for one freq, which also allowed for only 5-10 Khz either side you may be well out of range if you choose a different freq when buying. Especially if you didnt know what freq it was tuned to in the first place, as there was no crystal installed.

daveosoar

I think we have a definition difference!
All UK Txs are on the 35Mhz band. you can choose whatever 'channel' within the available spectrum, ie any pair of matching Xtals.
BUT on site the question "what channel" or "what frequency" are often synonomous . Sorry for any confusion!
Dave.

PlaneHeli

daveosoar,

Yes I think we are on the same wavelenght. I did mean you can changed to any CHANNEL you wish within that band.

I understand you cannot switch between 35 Mhz, 40Mhz 72 Mhz

This is the way I always understood it to be.

redfox said

You can't just change the Tx frequency, well you can but your taking risk in that you system won't be tuned for the channel your using and your range will be greatly reduced.

Vogen said

a change of 5kHz (probably even 10 or 20kHz) won't make that much of a difference in the radio system performance. However going from one end of the band to the other will probably cause issues. This is because the RF output stages are tuned to a particular frequency (ie the one that it is sold with) in the factory.

Now im confused.

NemisusDM

My Feedback: (3)

Hey Caliber,
Actually both are correct. We are an international group with a multitude of RC systems being used. Newer radio systems are made much tighter than the old ones. This is a double edged sword though. A tighter system will have a higher rejection to interferance but, large changes from center tuned frequency reduce performance on both the TX'er and RX'er. The RX'er looseing sensitivity and image rejection can be more of a problem then the TX'er reduceing output power....
Again this is just one manufacturer and in a specific market region that has to meet FCC regulations here in the US. In Australia I think they follow the ACMA regulations and in the UK I think it's OFCOM.

From the Futaba site I got this......

""Please explain Hi and Low band to me? I am not sure what the difference is between them.

Most of our 72Mhz receivers are now tuned to the center of either a low band frequency (channel 11 to channel 35) or high band (channel 36 to channel 60).

By tuning to each band, you can easily and safely change your receiver crystal to anywhere within this high or low band range, without having to send it in to be retuned. ""

PlaneHeli

Thanks Nemisis,

I see how it works now. I remember reading about high and low band but thought they were comparing for example 35Mhz with 72Mhz. Not the frequency within the band.

So I can safely use any frequency within my band.

Ive done several searches on the internet, but most sites are for the US. Even my manual only talks about the US and the allowed freqs been 72. However mine is 40mhz, and I cant find anywhere any info stating it has Hi and Lo bands.

Perhaps that is because there are fewer chs available on the 40Mhz band, whereas the 72Mhz band in the US appears to have far more chs.

Cheers cal83