[newbie] Inverted V antenna installation - high SWR?
 

Hi,

I am a new radio amateur from India (I got my license only a couple of
months back). My callsign is VU3RDD. This is my first experience with
Radio installation, so the questions and the problem I am facing may be
too silly and stupid. Neverthless... here it goes.

I live in an apartment on first floor. My inverted V for 40m and 20m
are on concrete terrace (of the 5 floor apartment). The mast is about
10 ft high. The ends of the dipole are not very symmetrically tied, as
the space constraints do not permit so. One of the ends of the dipole
is facing north and the other end towards west. Not exactly 90 degree
wide, but probably 100 to 120 degrees wide. The hight of the ends from
the terrace is not the same. I purchased a used rig last week and when
I measured the SWR, it sometimes hits 2:1, and sometimes more (if I
shout at the mic) for both 20m and 40m.

I have afew questions.

1. What are the ways to improve my antenna installation?

2. Does the concrete terrace act as ground for the antenna, and is 10
ft mast, just too short a height. The terrace itself is at around 50 ft
from the Ground.

3. I run a low loss RG213 coax from the terrace. Does the loss in the
cable contribute to the high SWR I am seeing?

Any other suggestions or general observations about this setup and hopw
I can improve the antenna setup? I also plan to learn NEC and simulate
this setup (hope it is possible to do it with NEC).

Thanks

73
Ramakrishnan, VU3RDD

On 22 May 2005 22:35:37 -0700, "
wrote:

Hi,

I am a new radio amateur from India (I got my license only a couple of
months back). My callsign is VU3RDD. This is my first experience with
Radio installation, so the questions and the problem I am facing may be
too silly and stupid. Neverthless... here it goes.


Hi Ramakrishnan,

Welcome to the fraternity. What you describe are common enough
problems and part of the "fun" of the hobby. Antennas are the last
frontier (which is to say you have to spend a lot of money to buy one
that works as advertised - and only if you spent far more to obtain a
tower).


I live in an apartment on first floor. My inverted V for 40m and 20m
are on concrete terrace (of the 5 floor apartment). The mast is about
10 ft high. The ends of the dipole are not very symmetrically tied, as
the space constraints do not permit so. One of the ends of the dipole
is facing north and the other end towards west. Not exactly 90 degree
wide, but probably 100 to 120 degrees wide.

This is fine.

The hight of the ends from
the terrace is not the same.

No problem with this either.

I purchased a used rig last week and when
I measured the SWR, it sometimes hits 2:1, and sometimes more (if I
shout at the mic) for both 20m and 40m.

This should really be done in CW mode, not voice mode.

I have afew questions.

1. What are the ways to improve my antenna installation?

By any number of ways, but what you have is in many ways far ahead of
the general Ham. That is, you have an asset with more than adequate
height for the bands you describe.

2. Does the concrete terrace act as ground for the antenna, and is 10
ft mast, just too short a height. The terrace itself is at around 50 ft
from the Ground.

I presume you mean, by mast, that it is 10 feet out from the terrace
which is 50 feet above ground. That is all fine and well, but you say
nothing of the dipole ends. That is, their height above ground. It
shouldn't matter too much, unless they are planted into the ground.

3. I run a low loss RG213 coax from the terrace. Does the loss in the
cable contribute to the high SWR I am seeing?

No, loss if anything will tend to present a lower appearance to a high
SWR situation. For a lossless line, SWR is constant all along its
length (which means you would measure the same SWR anywhere between
the transmitter and the mismatched load).

Any other suggestions or general observations about this setup and hopw
I can improve the antenna setup?

I don't see any discussion of a tuner. You will eventually need one,
if you don't need it already.

I also plan to learn NEC and simulate
this setup (hope it is possible to do it with NEC).

This is ambitious and commendable. As I said, antennas being the last
frontier makes this the last chance of experimentation (other than
getting the hang of using your gear).

73's
Richard Clark, KB7QHC

Hi Richard,

Richard Clark wrote:
Welcome to the fraternity. What you describe are common enough
problems and part of the "fun" of the hobby. Antennas are the last

Thanks. I hope to spend a lot of time, on learning and working on
antennas.

I presume you mean, by mast, that it is 10 feet out from the terrace
which is 50 feet above ground. That is all fine and well, but you
say
nothing of the dipole ends. That is, their height above ground. It
shouldn't matter too much, unless they are planted into the ground.

One end of the dipole is at about 4 ft above the terrace (which is 50
ft
from ground) and the other at about 6 ft from terrace.

I don't see any discussion of a tuner. You will eventually need one,
if you don't need it already.

I don't have a tuner yet. It's directly connected to an antenna.

This is ambitious and commendable. As I said, antennas being the
last
frontier makes this the last chance of experimentation (other than
getting the hang of using your gear).

Thanks Richard for the kind and encouraging words.

What could be the possibl;e reasons for the high SWR I am seeing?

73
Ramakrishnan, vu3rdd

wrote in message
ups.com...
Hi,

I am a new radio amateur from India (I got my license only a couple of
months back). My callsign is VU3RDD. This is my first experience with
Radio installation, so the questions and the problem I am facing may be
too silly and stupid. Neverthless... here it goes.

I live in an apartment on first floor. My inverted V for 40m and 20m
are on concrete terrace (of the 5 floor apartment). The mast is about
10 ft high. The ends of the dipole are not very symmetrically tied, as
the space constraints do not permit so. One of the ends of the dipole
is facing north and the other end towards west. Not exactly 90 degree
wide, but probably 100 to 120 degrees wide. The hight of the ends from
the terrace is not the same. I purchased a used rig last week and when
I measured the SWR, it sometimes hits 2:1, and sometimes more (if I
shout at the mic) for both 20m and 40m.

I have afew questions.

1. What are the ways to improve my antenna installation?

2. Does the concrete terrace act as ground for the antenna, and is 10
ft mast, just too short a height. The terrace itself is at around 50 ft
from the Ground.

3. I run a low loss RG213 coax from the terrace. Does the loss in the
cable contribute to the high SWR I am seeing?

Any other suggestions or general observations about this setup and hopw
I can improve the antenna setup? I also plan to learn NEC and simulate
this setup (hope it is possible to do it with NEC).

Thanks

73
Ramakrishnan, VU3RDD


Have you checked the SWR at different points in the bands? If you have what
differences have you measured. The antenna might just need tuned.. without
knowing how the difference between the top and bottom of the band it's hard
to say if anything else is affecting it.

Adair

Hi Adair,

I will try doing that. The said SWR is at around middle of 20m and 40 m
bands.

ramakrishnan

On 22 May 2005 23:47:27 -0700, "
wrote:

I don't have a tuner yet. It's directly connected to an antenna.

resolves:

What could be the possibl;e reasons for the high SWR I am seeing?

Hi Ramakrishnan,

The why of your question is simple: the antenna is not tuned to the
frequency you are applying. An alternative is that the set's built-in
SWR meter is hardly a precision instrument - another reason for an
external tuner (with a larger readable SWR meter).

But back to the antenna not being tuned. Again, this is the thrill of
"the last frontier." There are MANY variables that contribute to
tuning and it takes only one variable to be off, and so is the tune.
You should count it as great luck to have been so close to tune with
your first try.

Tune through all frequencies that are available to you, and build a
chart of SWR vs. Frequency. Then make a change in the antenna. This
can include
* changing the angle of the V
* changing the length of either leg of the V
* raising or lowering either/both leg(s) of the V
* raising or lowering the feedpoint of the V
and for each variation taking readings for new charts of SWR vs.
Frequency. Note the trends as they will reveal where your design
focus should be heading.

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."

Well, as you can see the discussion has barely warmed up and you have
the prospects of starting a journal dedicated to all the variations of
a simple design. One maxim to observe:
"No one ever builds ONE antenna."
You may as well get used to the idea that nothing is permanent, even
if you get it right. Plan on being flexible so that doing things
repeatedly does not become a chore. If you note all the variations
listed above, it should suggest that at a minimum you should invest in
pulleys. ;-)

73's
Richard Clark, KB7QHC

Thanks Richard for the new frontiers!! :-) I am learning a lot.

Mine is a center fed dipole with a 1:1 balun which feeds both 40m and
20m dipoles. The ends are at unequal heights because I couldn't find a
convenient way to tie the ends in the terrace.

My next experiment is to measure SWR at dofferent frequencies. To make
things simple, is there some kind of input signal suggested? Shouting
can easily come down afew decibels, after going up and down the stairs
adjusting the dipole angle and lengths. :-) I don't have a CW key yet.
:-( So what's the other option?

73
Ramakrishnan, vu3rdd

One more question:

While measuring the SWR, the needle fluctuates, as the voice is
modulated. Is the "value of SWR" the highest it hits? What's the
convention?

73
Ramakrishnan, vu3rdd

wrote:
What could be the possibl;e reasons for the high SWR I am seeing?

2:1 is NOT a high SWR and is tolerated well by most
transceivers. The reason that the SWR is not 1:1 is
that the feedpoint impedance of the antenna is
something other than 50 ohms.
--
73, Cecil http://www.qsl.net/w5dxp

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Richard Clark wrote:

" wrote:
What could be the possibl;e reasons for the high SWR I am seeing?

The why of your question is simple: the antenna is not tuned to the
frequency you are applying.

There's no guarantee that a tuned antenna is going to have
a feedpoint impedance of 50 ohms. Between 0.1WL and 0.33WL
in height, the same resonant dipole can have a feedpoint
impedance variation between ~20 ohms and ~100 ohms. A resonant
dipole in free space will have an SWR of ~1.5:1 using RG-213.

The reason that the SWR for any antenna is not 1:1 is that
the load (the feedpoint impedance of the antenna) is not equal
to Z0 (the characteristic impedance) of the feedline.
--
73, Cecil http://www.qsl.net/w5dxp

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wrote:
I don't have a CW key yet. :-( So what's the other option?

You can generate a CW signal using FM mode and the mic
button if you guarantee absolutely no background noise,
i.e. absolutely no modulation. My SSB mic element failed
me one time and I got a CW message through without a CW
key by using the mic button in FM mode on my IC-706.
Please don't ask what my CW-WPM was. :-)
--
73, Cecil http://www.qsl.net/w5dxp

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wrote:
While measuring the SWR, the needle fluctuates, as the voice is
modulated. Is the "value of SWR" the highest it hits? What's the
convention?

Depends on the inertial dampening of the meter needle.
If you can, use FM mode with zero modulation or simply
plug in a shorted audio plug to your CW key socket.
--
73, Cecil http://www.qsl.net/w5dxp

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wrote
While measuring the SWR, the needle fluctuates, as the voice is
modulated. Is the "value of SWR" the highest it hits? What's the
convention?
______________

Probably the ratio of forward to reflected power in your antenna system does
not change with the power applied to it. But many tx circuits that measure
SWR must be manually calibrated for the forward power in the system in order
for an ~ accurate indication of SWR.

The best accuracy is possible using CW output, after calibrating the SWR
meter for that forward power level. As the average power during voice
modulation usually is less than the rated average power capability of the
tx, the varying SWR readings you see when voice modulating might all be
lower than the true SWR of the antenna system.

RF

On 23 May 2005 02:11:46 -0700, "
wrote:

Shouting
can easily come down afew decibels, after going up and down the stairs
adjusting the dipole angle and lengths. :-) I don't have a CW key yet.
:-( So what's the other option?

Hi Ramakrishnan,

Give us some more details, like what rig you are using. There is a
chance that it has a "key" button for tune-up already. If you don't
have a CW key, I'm sure you have a screwdriver that would fit into the
jack (#2 Phillips maybe?).

So, as I understand it, you already have a "fan dipole." With uneven
length elements because of their tie-off, you could simply tie-off a
normal length element wire, and let the excess hang down. Take care
to select a tie-off point that is remote from surfaces (this may mean
that more wire hangs down, but this doesn't not seem to be an issue
with your height advantage).

Where in India are you located?

73's
Richard Clark, KB7QHC

On 23 May 2005 04:49:08 -0700, "
wrote:
While measuring the SWR, the needle fluctuates, as the voice is
modulated. Is the "value of SWR" the highest it hits? What's the
convention?

Hi Ramakrishnan,

This is why you need to do it with CW, and why you need an external
meter/tuner. Building a meter is actually quite simple (although
recent correspondence here would contest that statement).

SWR should not vary. It is dependant upon the mismatch of the
transmitter to the load alone. When you get variations of SWR
readings depending upon signal strength, the problem is often an issue
of the meter, and then, secondarily, the transmitter's source
resistance. This is why you should tune at the level you are going to
transmit at. You can first get into the neighborhood with lower
levels while tuning (although you lack a tuner) and then boost to the
anticipated power for the last adjustment.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote
On 23 May 2005 04:49:08 -0700,
wrote:
While measuring the SWR, the needle fluctuates, as the voice is
modulated. Is the "value of SWR" the highest it hits? What's the
convention?

SWR should not vary. It is dependant upon the mismatch of the
transmitter to the load alone. When you get variations of SWR
readings depending upon signal strength, the problem is often an issue
of the meter, and then, secondarily, the transmitter's source
resistance.
_____________

'SWR meters' don't measure SWR directly. They sample the forward and
reflected signals, both of which vary during SSB voice modulation -- even
when the ratio between them (SWR) remains constant.

A variation in forward & reflected readings during SSB modulation is a
normal situation, and not necessarily traceable to the SWR meter, the tx
source Z, a varying antenna system Z, or anything else.

RF

Hi Richard,

I am using Icom IC-706 (not the newer one, but the oldest one), got it
from another local ham for a good price. I am using an external SWR
meter, of a friend. (Lafayete is the brand name I can read in the box).

I am located in Bangalore, a city located in the southern part of
India.

73
Ramakrishnan, VU3RDD
http://www.hackGNU.org

I found from the manual that I can do CW with the Icom supplied mic.
Wanted to try it yesterday evening, but because of heavy wind and rain,
there was (is still) no power for the whole of yesterday night untill
now. Hopefully today evening I will try it out.

I am scared about transmitting, as costly stuff like a transceiver can
be difficult to get repaired in India. Sending it abroad for repair
costs as much as I would pay for a new transceiver!

73
Ramakrishnan, VU3RDD
http://www.hackGNU.org/

On 23 May 2005 21:09:39 -0700, "
wrote:

I am using Icom IC-706 (not the newer one, but the oldest one), got it
from another local ham for a good price. I am using an external SWR
meter, of a friend. (Lafayete is the brand name I can read in the box).

Hi Ramakrishnan,

This rig is sure feature loaded. If you don't have the manual, visit:
http://www.icomamerica.com/support/m...c-706mkiig.pdf
As I suspected, a #2 Phillips will do the job of key-down. I didn't
take the time to wade through the manual - my taste in gear runs
towards surface mount meaning a single component that can be held
between two fingers, and the markings can still be read without heavy
lensing.

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

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?
If so, we can proceed along those lines too.

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

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.

73's
Richard Clark, KB7QHC

On 23 May 2005 21:15:25 -0700, "
wrote:

I found from the manual that I can do CW with the Icom supplied mic.
Wanted to try it yesterday evening, but because of heavy wind and rain,
there was (is still) no power for the whole of yesterday night untill
now. Hopefully today evening I will try it out.

I am scared about transmitting, as costly stuff like a transceiver can
be difficult to get repaired in India. Sending it abroad for repair
costs as much as I would pay for a new transceiver!

Hi Ramakrishnan,

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
Ruskis are still building tubes, it seems, so at least surface
shipping shouldn't cost as much there.

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.
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).

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

73's
Richard Clark, KB7QHC

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

Richard Clark wrote:
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.

Yes, there are four wires. Ok. Now I understand, it is called a Fan
Dipole. Thanks.

Okay. I did afew simple tests this morning. With output power at the
lowest level (L - on IC-706, I guess this may output 5 W probably), I
measured the SWR at various points at 14 and 7 Mhz. For 7 Mhz, I had to
increase the power, as the SWR meter needle couldn't achieve full swing
when measuring FWD power. I used CW as input. The SWR seem to be around
1.25 to 1.5 for higher frequencies (for almost all of 14 mhz), but
touches very high values 4 and above on 7 Mhz. I used around 20W for 7
Mhz to achieve the needle full swing. By the time, I finished the 7 mhz
tests, the radio got heated up. I suspect, that there can be an open
circuit at the balun-antenna connection for 40m dipole wires(?). I am
not sure.. Anyway, I am going to rework the entire antenna installation
this weekend or next. Too bad, I cannot do it before. :-(

I take this whole issue of SWR mismatch as a blessing in disguise, as
it prompted me to read a lot on dipoles, transmision lines, baluns and
general antenna installation decisions.

73
Ramakrishnan, vu3rdd

On 24 May 2005 21:47:45 -0700, "
wrote:

For 7 Mhz, I had to
increase the power, as the SWR meter needle couldn't achieve full swing
when measuring FWD power. I used CW as input. The SWR seem to be around
1.25 to 1.5 for higher frequencies (for almost all of 14 mhz), but
touches very high values 4 and above on 7 Mhz. I used around 20W for 7
Mhz to achieve the needle full swing.

Hi Ramakrishnan,

Sounds like 40M needs work. The SWR values for 20M were spot on.
Well, when you get to it this weekend, you should be aware that for a
fan dipole, you should adjust the lower frequency band first. This is
because its tune will cast an effect into the higher frequency dipole
(because of near proximity - otherwise known as coupling and their
harmonic relationship). Undoubtedly this tuning of 40M is going to
upset what appears to be a well tuned 20M dipole. Such is life.

Likewise, tuning the 20M side will cast an effect into the lower
frequency dipole, but to a lesser degree. Several back-and-forths and
the job should be done.

Again, the lesson of flexibility and anticipating these ups and downs
can reduce the pain of repetition.

By keeping the two dipoles' ends further separated, you can reduce
this coupling. However, I am sure this is dictated by the anchor
points that support your antennas. You can allow the 20M to droop
further away from the higher span of the 40M to accomplish this -
several feet is a useful guide suggested here, although others might
suggest less.

By the time, I finished the 7 mhz
tests, the radio got heated up. I suspect, that there can be an open
circuit at the balun-antenna connection for 40m dipole wires(?).

No, this "heating up" was a product of what I described earlier. If
your balun-antenna connection works so well for 20M, it isn't going to
limp along for 40M. A 1:1 W2DU style BalUn is very robust over the
majority of HF, and certainly for these bands.

73's
Richard Clark, KB7QHC