I raised the following questions on eHam forum and was given the following answers. FYI
1-5 of 5 messages Page 1 of 1
Icom 706MKIIG Reply
by KC2VOB on September 7, 2009 Mail this to a friend!
Hi All,
Why is it that multi-band radios have very little output power on 2m/440?
If not illegal, is it possible to modify IC-706MKIIG to put out 100w on 2m and about 50w on 70cm? It's ok with me if that would result in lesser power on HF.
Is it ok to keep a radio on top of power supply?
73,
KC2VOB
RE: Icom 706MKIIG Reply
by N3OX on September 7, 2009 Mail this to a friend!
"is it possible to modify IC-706MKIIG to put out 100w on 2m and about 50w on 70cm?"
No, of course not. If that could be done, it WOULD have been done... manufacturers don't hobble their PA sections by 3dB or 4dB arbitrarily. In fact, if anything, they probably push the devices a little hard and accept a little more distortion than they maybe should. 100W 2m / 50W 70cm would be a huge selling point.
The reason why these rigs have lower power output on VHF/UHF is because it's harder and more expensive to do higher output efficiently on VHF/UHF than it is on HF. You have to consider current draw and thermal issues... you have to worry about having too much VHF/UHF gain in a small box because you can get oscillations.
Amp or antenna are your options for more oomph...
73
Dan
RE: Icom 706MKIIG Reply
by N3OX on September 7, 2009 Mail this to a friend!
"Is it ok to keep a radio on top of power supply? "
Sure, more or less. The only issue is that it makes the ambient temperature around the radio higher. I doubt it puts it out of spec though...
RE: Icom 706MKIIG Reply
by KZ1X on September 7, 2009 Mail this to a friend!
Q. Why is it that multi-band radios have very little output power on 2m/440?
A. I think what you really men was "why do rigs like the IC706 put out less power on VHF/UHF than they do on HF?" and the answer is, in general, cost. To generate the same 100W of RF power, in the frequency range you mention, would make the rig much more costly. And another reason: you don't need the power ... read on ...
Q. If not illegal, is it possible to modify IC-706MKIIG to put out 100w on 2m and about 50w on 70cm? It's ok with me if that would result in lesser power on HF.
A. Well, you could always add on another external amplifier if you wanted to. It's not illegal for an amateur to modify a radio to do whatever you want, as long as emissions standards are adhered to, it's just not practical to do as you suggest. First off, the gain of the output devices falls off with increasing frequency (it's a matter of physics) and then there are packaging issues, dissipation, coupling, bandpass ... it would require a wholesale redesign of the radio.
Moreover, the real issue is, RF transmitter power isn't the limiting factor in your station performance. That limit is substantially set by your antenna. On HF, getting gain can be tough, a 20 meter Yagi is pretty big! so 100 watts to a smaller antenna with a gain of 1 results in 100 watts effective radiated power. Good for HF work, where the tropospheric propagation does not predominate. The same size antenna on 2 meters would have perhaps 16 dB gain. So if you put in 50 watts into that antenna, your effective radiated power would be ...
... 1214 watts !!!
So, as you can see, transmitter power is only a small part of how well your station works. Don't forget: your receiver is also a part of the station. There's little point transmitting to someone you cannot hear.
Q. Is it ok to keep a radio on top of power supply?
A. There are three issues here, that I see. The big one is thermal ... would doing as you suggest limit the cooling of either the power supply or the rig? Further, do you want to add to the heat of the radio by putting it next to a power supply which also has dissipation? The next issue is magnetic fields. Some rigs are sensitive to the strong magnetic field of a nearby linear power supply, if you are using one. Lastly, ergonomics play a role as well. Would access to the rig's controls be affected by placing the rig above the table height?
RE: Icom 706MKIIG Reply
by WG7X on September 7, 2009 Mail this to a friend!
It is very simple:
Most RF transistors develop less power at higher frequencies. Transistors that make more power on VHF and UHF are more expensive. That's why most manufactures use transistors that don't make a lot of power at VFH/ UHF.
That is also why you have separate connectors for the HF/ Six meter and VHF/UHF on the 706. There are two sets of finals in that rig, one for HF and low VHF the other for VHF/ UHF.
Do yourself a favor: If you really need more power than the rig alone can provide acquire an external amplifier. Monkeying with the finals in the radio will only bring you grief and expensive repair bills. Short of an external amplifier there is NO WAY that the 706 will ever come close to the power levels you asked about.
When you begin to shop for and price out the VHF and UHF amplifiers, you will begin to understand why the makers did it the way they did. High power at VHF and UHF is expensive!
Than, maybe you will also ask yourself why you were so concerned about higher power in the first place...
High power at VHF and above is usually the domain of weak-signal work and not necessary or desirable in the repeater areas. If you were into weak signal work, you probably would never have asked that question, or if you did it would have been asked on a different forum in an entirely different way.
No trying to be difficult, just trying to help you understand why things are the way they are.
73 Gary
Page 1 of 1
