I am proposing putting up a flat top 75 meter dipole at a height of 48'.
I would like to use the antenna primarily on both the 75 and 40 meter bands
but not always. The antenna will have to be located almost 300 feet from
the shack so I intend to use home brew 500 ohm line made with #12 solid
copper conductors. The SWR on such a line will be quite high on both 75 and
40 and be all over the map on other bands. How about 160? My question is,
what kind of transmission line losses will be present with SWR's up to 10 to
1 or even greater? I have my trusty Johnson Viking Kilowatt Matchbox to do
the tuning in the shack.
My present antenna is a 60' flat top dipole over the shack at an average
height of 34'. There is approx. 70' of 450 ohm feed line going to the
tuner. The Match Box loads the antenna on all bands from 75 thru 10 meters
with excellent results and good feed line balance. I had to modify the
Match Box to get it to 75 by utilizing the entire link and adding series
tuning to it. Eznec reports an swr of over 100 to 1 on such an antenna on
75 but it gets out well. The #18 conductors have got to have hot spots with
my 200 watts of output power! So far I haven't set anything on fire!

Comments Please, Mike K6JSY

Assuming a VSWR of 10 to 1, reasonable for 80 meters, and assuming line
losses are negligible for open wire tuned feeders, also reasonable for a
first approximation, your line losses will be about -3 dB.

EZNEC indicates that you will also have a VSWR of about 10:1 on 40 meters.

Conclusion: you will experience about -3 dB loss [1/2 S unit] on both 75
and 40 when fed with 500 ohm line.

Deacon Dave, W1MCE
+ + +

Michael Efseaff wrote:

I am proposing putting up a flat top 75 meter dipole at a height of 48'.
I would like to use the antenna primarily on both the 75 and 40 meter bands
but not always. The antenna will have to be located almost 300 feet from
the shack so I intend to use home brew 500 ohm line made with #12 solid
copper conductors. The SWR on such a line will be quite high on both 75 and
40 and be all over the map on other bands. How about 160? My question is,
what kind of transmission line losses will be present with SWR's up to 10 to
1 or even greater? I have my trusty Johnson Viking Kilowatt Matchbox to do
the tuning in the shack.
My present antenna is a 60' flat top dipole over the shack at an average
height of 34'. There is approx. 70' of 450 ohm feed line going to the
tuner. The Match Box loads the antenna on all bands from 75 thru 10 meters
with excellent results and good feed line balance. I had to modify the
Match Box to get it to 75 by utilizing the entire link and adding series
tuning to it. Eznec reports an swr of over 100 to 1 on such an antenna on
75 but it gets out well. The #18 conductors have got to have hot spots with
my 200 watts of output power! So far I haven't set anything on fire!

Comments Please, Mike K6JSY

Michael Efseaff wrote:
I am proposing putting up a flat top 75 meter dipole at a height of 48'.
I would like to use the antenna primarily on both the 75 and 40 meter bands
but not always. The antenna will have to be located almost 300 feet from
the shack so I intend to use home brew 500 ohm line made with #12 solid
copper conductors. The SWR on such a line will be quite high on both 75 and
40 and be all over the map on other bands. How about 160? My question is,
what kind of transmission line losses will be present with SWR's up to 10 to
1 or even greater? I have my trusty Johnson Viking Kilowatt Matchbox to do
the tuning in the shack.

You might expect matched line loss of around 0.1dB for 300ft of that
type of line in dry conditions. The additional loss due to a VSWR of 10
would be around 0.4dB. The additional loss due to VSWR is probably quite
manageable in this case up to about 25:1 where it would be 1dB. Some
will argue, you could stand an S-point or two... you choose.

There will be losses in the tuner additional to the above, so factor
those into your loss budget.

My present antenna is a 60' flat top dipole over the shack at an average
height of 34'. There is approx. 70' of 450 ohm feed line going to the
tuner. The Match Box loads the antenna on all bands from 75 thru 10 meters
with excellent results and good feed line balance. I had to modify the
Match Box to get it to 75 by utilizing the entire link and adding series
tuning to it. Eznec reports an swr of over 100 to 1 on such an antenna on

You have implied VSWR=100 on the 450 ohm line that you are using, or are
you quoting the VSWR that would occur on a 50 ohm line?

75 but it gets out well. The #18 conductors have got to have hot spots with
my 200 watts of output power! So far I haven't set anything on fire!

Comments Please, Mike K6JSY

Michael Efseaff wrote:

I am proposing putting up a flat top 75 meter dipole at a height of 48'.
I would like to use the antenna primarily on both the 75 and 40 meter bands
but not always. The antenna will have to be located almost 300 feet from
the shack so I intend to use home brew 500 ohm line made with #12 solid
copper conductors. The SWR on such a line will be quite high on both 75 and
40 and be all over the map on other bands. How about 160?

Nope, the SWR will not be "quite high" on 75 and 40 or anything higher in
frequency. It will be "quite high" on 160. I use such an antenna for the
HF bands and the SWR on the 450 ohm ladder-line never goes higher than
15:1 which is acceptable because 450/15 = 30 ohms. Take a look at my
all-HF-band dipole on my web page.
--
73, Cecil http://www.qsl.net/w5dxp



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Dave Shrader wrote:
Assuming a VSWR of 10 to 1, reasonable for 80 meters, and assuming line
losses are negligible for open wire tuned feeders, also reasonable for a
first approximation, your line losses will be about -3 dB.

You assume line losses are negligible and then say line losses will be
about -3dB? Exactly where are those losses occurring?
--
73, Cecil http://www.qsl.net/w5dxp



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Hm.

All in the same sentence, that "line losses are negligible" and "line
losses will be about -3 dB"? I assume, of course that you mean 3 dB, not
-3 dB. But does this mean that you consider 3 dB to be negligible? And
how did you calculate that as the line loss?

Roy Lewallen, W7EL

Dave Shrader wrote:
Assuming a VSWR of 10 to 1, reasonable for 80 meters, and assuming line
losses are negligible for open wire tuned feeders, also reasonable for a
first approximation, your line losses will be about -3 dB.

EZNEC indicates that you will also have a VSWR of about 10:1 on 40 meters.

Conclusion: you will experience about -3 dB loss [1/2 S unit] on both 75
and 40 when fed with 500 ohm line.

Deacon Dave, W1MCE

I should have been more specific. The line loss, in a 500 ohm open wire
tuned feeder system, IF MATCHED, can be assumed to be negligible. The
presence of the additional losses caused by the amplitudes of the
reflected components at a VSWR of 10:1 will be about 3 dB.

Is 3 dB negligible? It's approximately 1/2 S unit. Whether or not 3 dB
is negligible depends on the individual station. I get one judgment if I
consider QRP and a different judgment if I consider QRO. For me, 3 dB on
75/40 is acceptable due to a single antenna limitation and 1 KW output.

Roy, thanks for challenging my 'loose language'.

DD

Roy Lewallen wrote:

Hm.

All in the same sentence, that "line losses are negligible" and "line
losses will be about -3 dB"? I assume, of course that you mean 3 dB, not
-3 dB. But does this mean that you consider 3 dB to be negligible? And
how did you calculate that as the line loss?

Roy Lewallen, W7EL

Dave Shrader wrote:

Assuming a VSWR of 10 to 1, reasonable for 80 meters, and assuming
line losses are negligible for open wire tuned feeders, also
reasonable for a first approximation, your line losses will be about
-3 dB.

EZNEC indicates that you will also have a VSWR of about 10:1 on 40
meters.

Conclusion: you will experience about -3 dB loss [1/2 S unit] on both
75 and 40 when fed with 500 ohm line.

Deacon Dave, W1MCE

Dave Shrader wrote:
I should have been more specific. The line loss, in a 500 ohm open wire
tuned feeder system, IF MATCHED, can be assumed to be negligible. The
presence of the additional losses caused by the amplitudes of the
reflected components at a VSWR of 10:1 will be about 3 dB.

Not with open-wire transmission line on 75m. It would be less than
one dB.
--
73, Cecil http://www.qsl.net/w5dxp



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You've still lost me. If the line loss is negligible when matched, even
a high SWR won't increase loss appreciably. That is, in fact, why people
use open wire line when the SWR is high. So I don't believe your 3 dB
figure is correct. Where did it come from? How was it calculated?

Roy Lewallen, W7EL

Dave Shrader wrote:
I should have been more specific. The line loss, in a 500 ohm open wire
tuned feeder system, IF MATCHED, can be assumed to be negligible. The
presence of the additional losses caused by the amplitudes of the
reflected components at a VSWR of 10:1 will be about 3 dB.

Is 3 dB negligible? It's approximately 1/2 S unit. Whether or not 3 dB
is negligible depends on the individual station. I get one judgment if I
consider QRP and a different judgment if I consider QRO. For me, 3 dB on
75/40 is acceptable due to a single antenna limitation and 1 KW output.

Roy, thanks for challenging my 'loose language'.

Total loss = 10*log((a^2-|rho|^2)/(a(1 - |rho|^2))

ARRL Antenna 18th edition, Chapter 24, Equation 15

Whe a = matched line loss = assumed 1
rho = 0.8181 reflection coefficient for VSWR = 10

+ + +

OOOPPPSSS!!! Math error!

Reviewing my calculations I made a major math error!! Glad I kept the
work sheet.

In the numerator I used rho^2 not (1 - rho^2).

Now I am glad I retired from Engineering. :-)

Thanks again Roy!!

Conclusion, after I failed my ALGEBRA course, sheepish grin, the
additional loss is zero within the stated assumptions. Which just proves
I made 'Much ado about Nothing'.

Deacon Dave, W1MCE

I'll say three Hail Mary's and three Our Father's, assuming I can still
count to three!!

+ + +

Roy Lewallen wrote:

You've still lost me. ... Where did it come from? How was it calculated?

Roy Lewallen, W7EL