Hi Richard,

Richard Clark wrote:
On 23 May 2005 21:09:39 -0700, "
wrote:
This rig is sure feature loaded. If you don't have the manual,
visit:
http://www.icomamerica.com/support/m...c-706mkiig.pdf

Yes, I got the user manual and the service manual from this link:

http://www.qsl.net/icom/manuals.html

I presume you don't have the optional tuner, otherwise we wouldn't be
talking about SWR.

Yes, I do not have a tuner.

The external SWR meter is good enough (does it agree with the ICOM
front panel indicator?). Do you have a dummy load to test it
against?

Yes, it agrees. I don't have a dummy load. From what I read in this
group and elsewhere, I think a dummy load is the first test gear I
should get. It will definitely show whether the coax is faulty or the
antenna.

Your weblog has crashed by the way.

Oh.. thanks. Will fix it this evening.

You should also be following the thread
"Laport's 'Radio Antenna Engineering' available"
and downloading a copy if you are not bandwidth restricted. Laport
is
one of the more accessible writers on the craft of antenna design.

Yes, I had been trying for the whole of today from
http://www.r-bonomi.com/cgi-bin/laport but unfortunately not been able
to get it. Would appreciate if someone put an http/ftp download
somewhere.

73
Ramakrishnan, vu3rdd
http://www.hackGNU.org

Hi Richard,

Richard Clark wrote:
And especially for such a small form factor. Most of the reviews
I've
seen have been quite positive. The few negatives were about
software.
However, I would point out that in regard to my last maxim about you
never building just one antenna; you never own just one rig. Get a
"beater" that you can get a soldering iron into without melting the
front panel at the same time. Maybe even one with (gasp) tubes. The

Thanks for the suggestions. Yes, I think I am slowly learning the fact
that I should have multiple radios and antennas in hand.

Also, invest in a decent battery (car battery size) using your power
supply as a float charger. Then you can even out the power
shortages.

Ok..

In this case, however, a tube rig may be stretching the limits of a
battery. In that case, think of 20-25 year old transistor rigs
(where
the ICs are TTL only). This stuff is easily field serviceable (I
know, because I've raised several from the dead).

Ok.


This is all part of the "flexibility" you should plan on, as I also
mentioned before.

Ok. Thanks for all the suggestions. So, now I have to do quite a lot of
work to get back on air with a "decent SWR".

73
Ramakrishnan, VU3RDD

On 23 May 2005 23:54:06 -0700, "
wrote:

front panel indicator?). Do you have a dummy load to test it
against?

Yes, it agrees. I don't have a dummy load. From what I read in this
group and elsewhere, I think a dummy load is the first test gear I
should get. It will definitely show whether the coax is faulty or the
antenna.

Hi Ramakrishnan,

Coax is not something that goes wrong with subtle error. Simple
continuity testing is quite often enough. However, a good load still
resolves full power issues.

Given the low power setting available from your rig, a corresponding
low power load can be built without too much trouble. If you have a
suitable multimeter, you can even extend the range of your SWR meter
to do low level measurements. Crack open that Lafayette (it has to be
a good 30 years old) and see how simple things can be. Identify the
RF section, as distinct from the DC conversion that drives the meter.
You will notice there are not too many parts involved and getting to
know what they are for will give you the momentum to take on the low
power challenge.

A tuner is no more difficult than a similar box with a gozinta and a
comesoutta, a multipole switch for a coil you wind, and a couple of
variable caps stolen from an old radio.

This, again, goes to flexibility. In this regard, knowing where an
old parts outlet is like having a treasure house. Not sure what
options you have in that regard in Bangalore.

73's
Richard Clark, KB7QHC

On 23 May 2005 23:56:43 -0700, "
wrote:

Ok. Thanks for all the suggestions. So, now I have to do quite a lot of
work to get back on air with a "decent SWR".

Hi Ramakrishnan,

Well, in fact, you are not in a bad position even when you see 2:1.
There are no brick walls in this game, and the rig will take care of
itself if you try to do something wrong. It has what is called an
automatic "fold-back" circuit that simply refuses to put too much
power into a mismatched load.

Most fold-back circuits start to kick in -gradually- at 2:1. They get
more aggressive as the mismatch climbs.

The point of this protection is to reduce the heat burden to the final
transistors when the power it is trying to deliver just won't go there
(this is called reflection from the load). The fold-back circuit
allows the transmitter to run in a reduced capacity that idles along
at that tolerable heat level.

For example:

You are trying to drive 100W into a matched load. The finals don't
want to live under any more than their existing heating of 60 to 80 or
more Watts (this is power that will NEVER get to the antenna anyway as
efficiency is not remarkably high for HF voice). In fact, after some
minutes your fan will engage to keep the temperature at a safe level.
The net result is that you radiate 100W and waste that 60+ Watts of
heat. This CAN become a problem if your environment is already hot -
if you review the specifications for rated power out, I am sure they
describe the environment as being 20 to 25 degrees C. Hotter
environments demand "de-rating" the allowable limits.

So, in a sense, what is being maintained is an equilibrium of the
maximum tolerable heat burden at the final transistors (there are also
side issues of frequency stability that are heat related, but these
issues are not destructive). So let's press the envelope with two
strained examples:

1. You are trying to drive 100W into a 2:1 mismatch. The finals are
still under the same burden of 60+ Watts of heat to generate that
drive, but the load refuses to accept all 100W and puts an additional
heat burden (the reflection coefficient) back upon the finals to the
tune of about 12%. Hence that original heat burden, plus this
mismatch, boosts temperatures to 72+ Watts. The fold-back senses the
returned power (this circuit is what is driving your SWR meter by the
way) and it drops the excitation level so that the 100W is depressed
to say 80W instead. The heat burden of inefficiency for 80W is
cooler, but with the reflected heat burden now around 8 - 10 Watts the
fold-back has juggled the books to maximize your power out, while
holding roughly the same heat level. In reality, no one notices
(unless you discover the fan is running longer now).

2. You are trying to drive 100W into a 5:1 mismatch. With this
mismatch, roughly half the power you are trying to transmit is being
reflected back to add heat to your finals. The fold-back is likely to
be quite aggressive about how much real power you can expect to make
it to this poor load.

Now, there are two forms of heat. The slow kind, like a clothes iron,
and the quick kind, like a lightning stroke. The fold-back circuit is
incapable of distinguishing the two, and the 5:1 scenario, with the
wrong phase angle (reactive instead of a simple resistive mismatch)
can lead to a voltage breakdown (the lightning heat) or catastrophic
thermal runaway. Hence, you may find that at such levels (5:1) that
the fold-back is complete instead of partial. Hopefully the fold-back
is faster than catastrophe.

Still and all, it doesn't pay to push the limits when the load is
known to be seriously out of whack. There is no indication of this in
your reports of SWR.

So standard advice, conventional wisdom, and law converge with: always
reduce power to a level consistent with reliable communication.

73's
Richard Clark, KB7QHC

Great Richard!!! Thanks a lot for the detailed explanation.

I downloaded Laport's "Antenna Engineering" book. Thanks Dave.

73
Ramakrishnan, vu3rdd

Richard Clark wrote:
http://www.hackGNU.org
Your weblog has crashed by the way.

It works now.

73 - VU3RDD

Richard Clark wrote:
I would point out, however, that antennas intended for even harmonics
(40M/20M) usually portend disaster. I suggest you research the net
for "fan dipole." That you do not report any such disaster suggests
that due to what you DO report (unequal length dipole legs) suggests
that you have is what is called an OCF (off-center feed) dipole.
This
makes yet another variable that tends to throw a spanner in the gears
of tuning. In this regard, you need to research the net for the
topic
of "feedpoint choke" or "W2DU style BalUns."

Richard,

Probably I missed telling the list that, the coax feeds a 1:1 balun
which is connected to the dipole. The dipole is center-fed, and balun
feeds both both 20m and 40m wires of the dipole.

wrote:
I am using Icom IC-706 ...

On the IC-706 you can use FM mode to generate a CW signal.
--
73, Cecil http://www.qsl.net/w5dxp

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wrote:
I am scared about transmitting, as costly stuff like a transceiver can
be difficult to get repaired in India.

The IC-706 has a power output control and the final is
protected by foldback circuitry.
--
73, Cecil http://www.qsl.net/w5dxp

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On 24 May 2005 05:02:12 -0700, "
wrote:

Probably I missed telling the list that, the coax feeds a 1:1 balun
which is connected to the dipole. The dipole is center-fed, and balun
feeds both both 20m and 40m wires of the dipole.

Hi Ramakrishnan,

If, by this description, you mean that there are four wires, two for
20M and 2 for 40M; then that is all good and well.

However, if you mean that there are two wires, one for 20M and one for
40M arranged as a dipole; then that is unbalanced and what I called an
OCF Dipole.

By your description of a slight mismatch, I will take it that you are
using four wires - a Fan Dipole. This convention is widely used and
suited to your purposes.

73's
Richard Clark, KB7QHC