Feeding 30 Mtrs into a 40 Mtr Dipole - Losses ?
 

I've got about a 70 foot +/- run of RG-8X that I use to feed my 40
meter dipole. I also use the same feedline and dipole through an
MFJ-949E tuner to operate on 30 meters with. Out of curiosity can
someone tell me about how much a a signal loss I've got compared to
having an antenna that's actually cut for 30 meters ?

Thanks in advance.

73 Gary K8IQ

A quick NEC model of a 66 ft center fed horizontal dipole, at 35 ft, above
an average ground, indicates an input impedance of 307 +j708 ohms at 10.1
MHz.

For 70 ft of RG8X, using the ARRL's DOS program "TL", the matched line loss
is 0.63 dB, and an additional loss of 5.2 dB, for a total line loss of 5.83
dB. For an input of 100 W the ERP is therefore about 25 W. The coax input
impedance is 17.3 +j78.4 ohms.

73,

Frank


"Gary" wrote in message
...
I've got about a 70 foot +/- run of RG-8X that I use to feed my 40
meter dipole. I also use the same feedline and dipole through an
MFJ-949E tuner to operate on 30 meters with. Out of curiosity can
someone tell me about how much a a signal loss I've got compared to
having an antenna that's actually cut for 30 meters ?

Thanks in advance.

73 Gary K8IQ

I've got about a 70 foot +/- run of RG-8X that I use to feed my 40
meter dipole.

Out of curiosity can
someone tell me about how much a a signal loss I've got compared to
having an antenna that's actually cut for 30 meters ?

Hi Gary, According to my calculations the feedline loss is around 90%, so if
you are running 100 watts, and the tuner is lossless, 90 watts are lost in
RG-8X. The good news is 10% efficiency is only 1-2 S units down from 100% on
the recieving end. The bad news is, no tuner is lossless.
You will have loss using a 40m dipole on 30m. Does that satisfy your
curiosity:-)?
73 Gary N4AST

Gary, since your data are so different from mine, you have to tell me where
I went wrong.

Thanks,

Frank


"JGBOYLES" wrote in message
...
I've got about a 70 foot +/- run of RG-8X that I use to feed my 40
meter dipole.

Out of curiosity can
someone tell me about how much a a signal loss I've got compared to
having an antenna that's actually cut for 30 meters ?

Hi Gary, According to my calculations the feedline loss is around 90%, so
if
you are running 100 watts, and the tuner is lossless, 90 watts are lost in
RG-8X. The good news is 10% efficiency is only 1-2 S units down from 100%
on
the recieving end. The bad news is, no tuner is lossless.
You will have loss using a 40m dipole on 30m. Does that satisfy your
curiosity:-)?
73 Gary N4AST

Gary wrote:
I've got about a 70 foot +/- run of RG-8X that I use to feed my 40
meter dipole. I also use the same feedline and dipole through an
MFJ-949E tuner to operate on 30 meters with. Out of curiosity can
someone tell me about how much a a signal loss I've got compared to
having an antenna that's actually cut for 30 meters ?

Thanks in advance.

For 30M ops I suspect that you'd be much better off if you temporarily
shorted the center conductor and the braid of the coax together and
turned the feedline + dipole into a top-loaded vertical. Build a
"shorting widget" out of an SO-239 in order to be able quick-change
bands (sort of). On 40M feed it as a dipole as usual. On 30M install
the "shorting widget" on the 8X 259 connector, feed the widget from the
"random wire" connection on the back of the 949E and tune up. Attach a
couple 25 foot radials to 949E ground post.

73 Gary K8IQ

w3rv

The loss on coax with that kind of a mismatch is around 90%. You are
better off with open wire feeds, especially since you have a tuner
available..even if it is MFJ ( Hi ).
An open wire fed 40 meter dipole will produce a fine multiband
antenna..take that to the bank!
Jim

There's no doubt that many, or most, people could reduce their power by
a factor of 10 and still communicate ok. But 10 dB isn't a trivial
amount ("only 1-2 S units").

On my Icom 730, a 10 dB reduction will reduce an S9 reading to S6, S7 to
S2, or S6 to below S1. Have you tried a 10 dB reduction in signal
strength on your receiver and seen how the S meter responds?

If you had 10 dB feedline loss and decided you needed to get it back by
increasing your antenna gain, you'd need a 7 element Yagi on a boom
considerably longer than 1 wavelength. I can see it now -- an
advertisement for a 7 element long-boom beam -- "One to two S-Units gain!"

Roy Lewallen, W7EL

JGBOYLES wrote:

I've got about a 70 foot +/- run of RG-8X that I use to feed my 40
meter dipole.


Out of curiosity can
someone tell me about how much a a signal loss I've got compared to
having an antenna that's actually cut for 30 meters ?


Hi Gary, According to my calculations the feedline loss is around 90%, so if
you are running 100 watts, and the tuner is lossless, 90 watts are lost in
RG-8X. The good news is 10% efficiency is only 1-2 S units down from 100% on
the recieving end. The bad news is, no tuner is lossless.
You will have loss using a 40m dipole on 30m. Does that satisfy your
curiosity:-)?
73 Gary N4AST

On 10 Dec 2004 06:30:32 -0800, "
wrote:

The loss on coax with that kind of a mismatch is around 90%. You are
better off with open wire feeds, especially since you have a tuner
available..even if it is MFJ ( Hi ).
grin
An open wire fed 40 meter dipole will produce a fine multiband
antenna..take that to the bank!
Jim

Thanks to all that replied. I knew it was probably somewhat lossy but
I didn't know it was as bad as has been pointed out. I'll have to
think about feeding it with open wire or twinlead when the weather is
more suitable here in Michigan.

73 and Thanks again.

Gary K8IQ

There's no doubt that many, or most, people could reduce their power by
a factor of 10 and still communicate ok. But 10 dB isn't a trivial
amount ("only 1-2 S units").

I didn't mean to imply it was "trival". I have a little 100 watt XCVR that I
use moblie and on field day that has a low power (7 watts) setting that I
sometimes use to reduce battery drain. The qso total drops, but it is not as
drastic as you would think with a 10 dB power reduction. Of course on low
power I only go back to the guys with big signals, to make that 10dB seem less
of a problem.
I guess that 10 dB loss is all relative to where it is and what would be
required to get it back.
73 Gary N4AST

Gary, since your data are so different from mine, you have to tell me where
I went wrong.


Hi Frank, I used one of Reg Edwards Dipole programs to get the line loss. At
the time that was the fastest way I could think of to get some numbers. His
program lets you describe the antenna, feedline, and frequency of interest and
it gives a number of operating conditions including line loss.
As far as where you went wrong? May have been me that was wrong. I think I
have TL program somewhere, but don't have time to look at the moment. I
checked the impedance of the antenna a couple of ways, and it agrees with your
calculation about 300+j700. The difference must be in the way the line loss is
calculated?
73 Gary N4AST

Hi Gary, ok, glad that some of the numbers agree. I used NEC-Win Pro to
calculate the antenna input Z.

In real terms there is only 4 dB between our numbers for transmission line
loss, so not really a great amount. I am familiar with some of Reg's
programs, and have found them in reasonable agreement with other methods,
but have not tried his transmission line program.

The fact is, whatever the loss, the use of an open wire transmission line
would reduce the losses to near zero -- then he would probably find his
tuner could not match it!

73,

Frank
(VE6CB)

"JGBOYLES" wrote in message
...
Gary, since your data are so different from mine, you have to tell me
where
I went wrong.


Hi Frank, I used one of Reg Edwards Dipole programs to get the line loss.
At
the time that was the fastest way I could think of to get some numbers.
His
program lets you describe the antenna, feedline, and frequency of interest
and
it gives a number of operating conditions including line loss.
As far as where you went wrong? May have been me that was wrong. I
think I
have TL program somewhere, but don't have time to look at the moment. I
checked the impedance of the antenna a couple of ways, and it agrees with
your
calculation about 300+j700. The difference must be in the way the line
loss is
calculated?
73 Gary N4AST

You know, Gary, there is another option which no one mentioned. Add a 30M dipole under the 40M job and use the same
feedline. You will likely have to adjust the length of the 40M wire slightly to return the VSWR to the present value,
but you will then have a good antenna for both bands. See any number of references about actual mechanical
construction.

--
Crazy George
Remove N O and S P A M imbedded in return address
"Gary" wrote in message ...
On 10 Dec 2004 06:30:32 -0800, "
wrote:

The loss on coax with that kind of a mismatch is around 90%. You are
better off with open wire feeds, especially since you have a tuner
available..even if it is MFJ ( Hi ).
grin
An open wire fed 40 meter dipole will produce a fine multiband
antenna..take that to the bank!
Jim

Thanks to all that replied. I knew it was probably somewhat lossy but
I didn't know it was as bad as has been pointed out. I'll have to
think about feeding it with open wire or twinlead when the weather is
more suitable here in Michigan.

73 and Thanks again.

Gary K8IQ

On Sat, 11 Dec 2004 10:14:26 -0600, "Crazy George"
wrote:

You know, Gary, there is another option which no one mentioned. Add a 30M dipole under the 40M job and use the same
feedline. You will likely have to adjust the length of the 40M wire slightly to return the VSWR to the present value,
but you will then have a good antenna for both bands. See any number of references about actual mechanical
construction.

Thanks for the idea George, I hadn't thought of that one. I knew the
30M into the 40 dipole wasn't the best in the world as I didn't hear
the signals on 30 that I would expect to hear but I didn't know it was
as lossy as pointed out. I'll have to give it a go this spring or
summer.

73 Gary K8IQ