To be serious your 80m SWR plot looks like my dipole. I cut mine so it’s centred around 3.6MHZ which means it’s OK for 3.560 CW QRP and the bottom of the allmodes and SSB sections. It may besub-optimal for whole band use but it’s for me activating so I simply pick frequencies that work for me.
I beg to differ. We’re discussing how the need (or not) for an external ATU makes the bulky IC-705 a less attractive solution compared to other rigs for small-volume backpack activations
Yeah, when I was trimming the length of the dipole it occurred to me that I was optimising it around frequencies that I don’t even use but necessary for low-ish VSWR at band ends. In any case, the KX2’s internal ATU easily gets 1:1.0 across the entire band.
[I can detect no insertion loss due to the internal ATU on any of my dipoles at their resonant frequency so usually leave the ATU switched in].
Thanks,
Good video. Makes the point it won’t suit everybody but no rig ever does. The KX3 according to some a bit fragile mechanically and VHF is an afterthought.
The KX2 is HF only but gee it’s a nifty size.
The FT817/8 has no ATU. It is very old technology now.
The IC705 has so many features that some of us can forgive the lack of a ATU.
The IC705 might be overkill for SOTA. My MTR3 has stood me in good stead, but it also needs an ATU and makes the KX2 look like bloatware size wise.
Horses for courses.
Don’t sneer at the other guy’s choice. He might think you are a nutter.
We all have different interests and wants.
Enjoy the diversity
73
Ron
VK3AFW
You clearly get better sales prices down under! I’d be pleasantly surprised if they dropped the price of it here by more than fifty quid in a sale. Of course, sometimes the adverts say “£phone” so you don’t know what price they’ll actually take until you call, and the vaguaries of exchange rates can knock prices every whichway (though more often up than down…). As a comparison, I think the lowest published price I’ve seen the IC-7300 for sale here was the launch price (1099.95 IRC?), and it didn’t stay there for long. Then it went up. Mind, there was a sharp drop in the value of the pound in June 2016 for some reason or other. . .
Except that it’s not really. The antenna is still mismatched to the transmission line. A network of inductors and capacitors in the radio can compensate for the reactive components of the load impedance presented to the radio and make it look like a resistive load that will keep the PA happy and able to operate at full power. You’ve still got standing waves on the transmission line and the SWR isn’t 1:1.
That’s not to say that the internal ATU isn’t useful - it is. But it isn’t magically fixing a high SWR on the transmission line between radio and antenna.
Martyn M1MAJ
3 Likes
Hi Martyn, I didn’t say or imply an ATU magically fixes anything (you make a strawman argument).
As you point out [and most of us know] an ATU tries - often successfully - to eliminate the reactive part of any impedance mismatch but that may still leave you with a resistive mismatch.
When the needle on my external power/s-meter is not even deflecting from the 1:1.0 mark [or it’s showing zero on the reflected power setting] I assume the resistive mismatch is very low.
My original points remain:-
- an ATU can improve the Tx-to-dipole match across the entire band which can make a significant performance difference for the QRP portable operator
- I would put an internal ATU much higher up the list of desirable features of a portable QRP rig than ‘bells and whistles’ like a touch screen.
regards, Andy
2 Likes
Sorry, I didn’t mean to imply that you had said that. I wasn’t thinking of this as an argument!
That is perfectly true, and often forgotten, but it wasn’t the point I was intending to make.
I agree that an internal ATU can be a very useful thing, enabling the transmitter to transfer full power to a load which does not present the ideal impedance.
My point, and I admit it’s a slightly pedantic one, is that it is not achieving a 1:1 SWR on the transmission line to the antenna. If the transmission line were lossless, a high SWR on it wouldn’t matter. But if you are concerned about the high SWR leading to feeder losses, an internal ATU won’t help with that.
There is a nominal 1:1 SWR on the connection between PA and ATU, but with an internal ATU that is so short compared with the wavelength that it could not support standing waves anyway!
Martyn
1 Like
Hello Colin, I agree totally with your comments about this rig.
73 de Geoff vk3sq
Well Geoff you and Colin would be candidates for a mcHF kit that you could transform into the ideal transceiver, with inbuilt ATU and a tx all day battery. Just requires a bit of building and software tweaking. Maybe add a PA?
Lots of them about
73
Ron
VK3AFW
We’re getting into a narrow technical discussion here [which is fine by me] and I hope other readers will forgive our relevant digression from the thread title [as opposed to being off topic].
We’re testing my very rusty recollection of transmission line theory here [Experts feel free to correct anything]. My understanding is: -
- We get maximum power transfer from source (Tx) to load (dipole) when both have the same resistance [and no reactance]. The transfer efficiency is at best 50%.
- The radiation resistance of a centre-fed, inverted-vee, half-wave dipole [as I use] falls from the nominal 75 ohms of a horizontal dipole to around 50 ohms (depending on angle of slope).
- I use a 50-ohm coax feeder. When making and testing the dipole I place the Power/S-meter between the Tx and the feeder as I’m interested in the VSWR of the antenna/feeder combination. [Sadly, I don’t have an antenna analyser]
- With no ATU enabled, I will get a U-shaped VSWR curve like the two graphs I posted earlier usually with a minimum at or close to 1:1.0 [within the resolution of the meter]. I then repeatedly trim a few inches off each wire leg until the minimum shifts up to the desired frequency.
- Your comment about standing waves confused me: are you suggesting a standing wave on the transmission line is desirable? [I think there is no net transfer of energy along the line with a standing wave. And we need a progressive (or travelling) wave to transfer energy from source to load.]
Andy
For those who want to look inside of ICOM-705.(in Japanese)
1 Like
The ATU lets the TX “see” 50R and so the TX is happy. The other side of the ATU is connected to your feeder. The impedance there is whatever it was, the ATU only keeps the TX happy. When the feeder is mis-matched to the load then there is mis-match loss along with normal loss per m length . The loss is much higher per m on coax than open wire/ladder line etc.
So the ATU keeps the TX happy but there will be some/lots/enormous loss on the feeder if it’s coax. If you are QRP you dont want say an extra mis-match loss on the feeder, you haven’t got any power to loose in the first place.
e.g.
10m of RG58 at 10MHz.
With 1.1:1 match
match loss to SWR/mismatch = 0.002dB
length loss = 0.437dB
total loss = 0.439db
10W drive = 9.038W at the antenna
With 10:1 match
match loss to SWR/mismatch = 1.365dB
length loss = 0.437dB
total loss = 1.802db
10W drive = 6.603W at the antenna
So if you have a mismatch in the antenna you don’t want coax feeder.
1 Like
I think that’s a special case of load impedance equal to the complex conjugate of the source impedance, but basically, yes.
Not at all. For any real transmission line, it will work best if terminated in its characteristic impedance.
There is no transfer of energy if the VSWR is 1:∞, i.e. there are points on the line where the voltage is zero - pure standing waves. That’s an extreme case - e.g. the line is open or short circuit at the far end.
In practice with a more modest VSWR there are travelling waves in both directions interfering with each other to generate a standing wave component. The VSWR is by definition the ratio of the largest and smallest voltages along the line (assuming it is at least half a wavelength long). If the line is lossless, all of the energy that goes in one end has to come out of the other (conservation of energy). It follows from this that the forward wave is actually carrying more power than you put in; the excess being what is being reflected back. So provided that the transmitter is happy to push the energy into the line (and you can make it happy with an ATU) all of the power that leaves the transmitter will reach the antenna. So the standing wave component is not harmful in the theoretical case of a lossless transmission line.
Real transmission lines obviously have losses. And those losses are made worse by the higher peak voltages and currents present if you have a standing wave component on the line.
My point, I think, is that whilst the internal ATU helps to get the power to flow through the socket on the back of the transmitter, it can’t do anything to reduce the losses in the transmission line that arise as a consequence of not having the entire system perfectly matched. That would be best achieved by terminating the transmission line with it’s characteristic impedance, and that can be done by matching at the antenna feed point.
Martyn
Hi,
Mostly agree but…
Many people have the wrong idea about the output resistance of the tx. For a 50 ohm load the maximum power out occurs when the load line is effectively 50 ohms. We do not want the resistance to actually be 50 ohms. Many class D PAs achieve near 90% efficiency. Class B ought to be circa 75% efficiency in bigger amplifiers.
If the PA resistance were really 50 ohms then 50% efficiency would be Utopian.
When the losses in the drivers, digital bits, etc are added to the PA losses the RF output of many transceivers may be as low as 30% of the DC input. Hence many think the PA is 50 ohms. In practice most PAs achieve 60+% in compact gear. It’s all the other stuff that makes the difference.
The load line of a 100 W HF PA is going to be less than 1.2 ohms and is stepped up via a transformer.
The load line is not a physical heat dissipating resistance. It is the zero current PA voltage divided by the saturation - minimum voltage - current.
73
Ron
VK3AFW
When I look through the FCC’s website, it doesn’t look like Icom has even started the process for FCC approval in the USA. Has anyone else checked it out? Am I missing something? It seems a bit odd that they haven’t even started the process yet.
Is the 705 approved yet anywhere other than Japan? Have they started the approval process anywhere?
73,
Tom, N2YTF
Indeed. When we say that a transmitter has a 50Ω output, we don’t mean that the source impedance is 50Ω. We mean that the designer intends it to drive a 50Ω load.
However I don’t think that affects the discussion of what happens on the transmission line downstream.
Martyn
2 Likes
I think an internal ATU would have been better than the rather small stock Li Ion battery they use. So many of us carry external LiPos, there doesn’t seem any point in having such an expensive Icom brand battery when it could have had a built in ATU.
73 de OE6FEG
Matt
1 Like
An ATU would be good if it were a pi/ell network and not the usual tee network. But it would be a lot bigger than the battery provided.
The battery is on a par with other batteries in small rigs. Big enough for a couple of activations. Having a standard battery means that you can change it easily.
It’s being touted as a full base station featured portable. Hard to confirm that from the brochure. No manual on line as of today. Certainly does a lot more than the venerable FT817.
Hardware expected in VK on 14/8/20.
73
Ron
VK3AFW
I’ve been running my KX2 at 3W recently and so far have 5 activations on one charge with juice left for more and 98 QSOs in the log. Individual 18650 cells are small and easily changed, but the reason you won’t get a 3 or 4 cell cradle in a radio rather than a stock battery is nothing to do with user-friendlyness. The low voltage of the supplied battery makes it harder to get 10W clean RF, something that is not a problem with non-stock batteries.
73 Matt