Transmission line loss

[Mike B-12]

First of all, I have to congratulate the posters in the earlier thread on line
loss, etc., for prompting me to actually get my calculator & ARRL Handbook out
and read up on transmission line loss, dB calculations, etc.

My interest has come up as I'm planning on installing a flag pole vertical in my
yard, due to CCRs, and the coax run will be around 100 feet outside the house,
plus 28 feet inside.

Here's my take on line loss, and please correct me if I'm wrong.  For today's
discussion, let's assume the power is 100 watts, total run is 100 feet, that I
can tune this vertical up to 50 MHz, and the SWR at the antenna hits 5:1 (it
won't, as I'll have a remote tuner there, but just for discussion...).

Davis RF-brand "RF-9914F Bury-Flex" has a published matched line loss of 1.1
dB/100 ft at 50 MHz.  Per the Handbook, a 5:1 SWR at the load will add around
1.2 dB of loss, for a total of 2.3 dB over the run, for about 60% efficiency.
(I'm still fuzzy on the percentage calculations.)

A variety of brands of RG-8X have a matched line loss of 2.0 dB at 50 MHz per
100 ft.  A 5:1 SWR adds 1.7 dB, for a total of 3.7 dB loss, for a little over
42% efficiency.

Am I reading something wrong here, or is the 1.4 dB difference between the two
cables correct?  Can you extrapolate that to an S-meter at 6 dB/S-unit?  If so,
I'd say the 1.4 dB would be barely noticeable on the receiving end at best, and
the extra cost of lower loss coax isn't worth the money.  Operation at HF would
be even less of an difference than at 50 MHz.

I realize there may be other considerations as to the type of coax used that
could sway one's decision.  In my case, I want direct buriable, and I haven't
come across any 8X that allows this.  Longer runs and higher SWR would obviously
sway the argument as well.

RG-6 seems to be in between the RG-8X and Bury-Flex as far as loss, but the cost
may be much better (from what I've read here).

In my searching, I came across a pretty handy document from Cisco relating to dB
loss and gain:
http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a00800e90fe.shtml
Thanks for your input.

73,

Mike
KW1ND
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[Don Wilhelm-4]

Mike,

Take a look at the loss from 0.40 inch diameter 75 ohm coax that may be
available free as cutoffs from your cable installing company - it is
burial rated and will often outperform the 50 ohm coax when you do the
loss calculations.

You are correct that the matched line loss will vary with frequency, but
the added loss due to SWR is independent of frequency - it only depends
on the SWR and the matched line loss.

73,
Don W3FPR

Mike B wrote:

> First of all, I have to congratulate the posters in the earlier thread on line
> loss, etc., for prompting me to actually get my calculator & ARRL Handbook out
> and read up on transmission line loss, dB calculations, etc.
>
> My interest has come up as I'm planning on installing a flag pole vertical in my
> yard, due to CCRs, and the coax run will be around 100 feet outside the house,
> plus 28 feet inside.
>
> Here's my take on line loss, and please correct me if I'm wrong.  For today's
> discussion, let's assume the power is 100 watts, total run is 100 feet, that I
> can tune this vertical up to 50 MHz, and the SWR at the antenna hits 5:1 (it
> won't, as I'll have a remote tuner there, but just for discussion...).
>
> Davis RF-brand "RF-9914F Bury-Flex" has a published matched line loss of 1.1
> dB/100 ft at 50 MHz.  Per the Handbook, a 5:1 SWR at the load will add around
> 1.2 dB of loss, for a total of 2.3 dB over the run, for about 60% efficiency.
> (I'm still fuzzy on the percentage calculations.)
>
> A variety of brands of RG-8X have a matched line loss of 2.0 dB at 50 MHz per
> 100 ft.  A 5:1 SWR adds 1.7 dB, for a total of 3.7 dB loss, for a little over
> 42% efficiency.
>
> Am I reading something wrong here, or is the 1.4 dB difference between the two
> cables correct?  Can you extrapolate that to an S-meter at 6 dB/S-unit?  If so,
> I'd say the 1.4 dB would be barely noticeable on the receiving end at best, and
> the extra cost of lower loss coax isn't worth the money.  Operation at HF would
> be even less of an difference than at 50 MHz.
>
>  

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[N2EY]

In a message dated 11/17/07 12:02:32 AM Eastern Standard Time,
[hidden email] writes:


> For today's
> discussion, let's assume the power is 100 watts, total run is 100 feet, that
> I
> can tune this vertical up to 50 MHz, and the SWR at the antenna hits 5:1 (it
> won't, as I'll have a remote tuner there, but just for discussion...).

For comparison, check out the online loss calculator at:

http://www.ocarc.ca/coax.htm

> Davis RF-brand "RF-9914F Bury-Flex" has a published matched line loss of
> 1.1
> dB/100 ft at 50 MHz.  Per the Handbook, a 5:1 SWR at the load will add
> around
> 1.2 dB of loss, for a total of 2.3 dB over the run, for about 60%
> efficiency.
> (I'm still fuzzy on the percentage calculations.)

According to the online calculator, the matched loss of that coax is 1.009
dB/100 ft.  It shows the additional loss due to 5:1 SWR as 1.13 dB, so the total
loss is 2.139 dB and about 62% efficiency. Insignificant difference between
your calculations and the online calculator. .

> A variety of brands of RG-8X have a matched line loss of 2.0 dB at 50 MHz
> per
> 100 ft.  A 5:1 SWR adds 1.7 dB, for a total of 3.7 dB loss, for a little
> over
> 42% efficiency.

The online calculator shows Belden 9258 (which is RG-8X type) as having
slightly higher matched loss (2.188 db/100 ft @ 50 MHz) which makes the SWR loss
1.784 dB and the total loss 3.972 dB. Efficiency almost exactly 40%. Again, an
insignificant difference between your calculations and the online calculator. .

>
> Am I reading something wrong here, or is the 1.4 dB difference between the
> two
> cables correct?  Can you extrapolate that to an S-meter at 6 dB/S-unit?  If
> so,
> I'd say the 1.4 dB would be barely noticeable on the receiving end at best,
> and
> the extra cost of lower loss coax isn't worth the money.  Operation at HF
> would
> be even less of an difference than at 50 MHz.

All the numbers I have are close to the numbers you got. But there are other
factors to consider.

First is the actual coax length. IIRC, you need 128 feet, not 100 feet, and
that pushes the losses up a bit. In practice the coax length will probably need
to be greater - maybe 150 feet when all is said and done. For example, you
want to leave some slack at both ends of a buried cable.

Second is the cost of a dB. If the difference between a K3/10 and a K3/100 is
ten dB and the price differential is $380, that's $38 per dB  - but only on
transmit. If better coax reduces the overall loss by, say, 2 dB, and the cost
differential is $50, that's $25 per dB. And it works on both transmit and
receive, not just transmit.

Remember too that a lot of the 'cost' of the installation will be in the work
it takes to put it up.

Third is maxing out performance. While an S-unit of loss will normally not be
noticeable most of the time, when things get tough it can be the difference
between a QSO and no QSO. This is one big reason the big guns work stations the
rest of us don't.

Fourth is purely esthetic - some folks like knowing that they have the
highest efficiency reasonably possible.

73 es GL de Jim, N2EY


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[Doug Person]

The bigger difference between RG-8X and 9914 is power handling
capacity.  Also, there are some low-loss RG-8X cables out there that cut
the loss difference by half again.  I bought a 500' spool at a hamfest
some years ago.  It's double shielded.  I've used short runs to UHF
antennas for satellite work with good results. But, no doubt that the
9914 Bury-Flex is among the best of the half inch flexible cable money
can buy.

Doug -- K0DXV

Mike B wrote:

> First of all, I have to congratulate the posters in the earlier thread on line
> loss, etc., for prompting me to actually get my calculator & ARRL Handbook out
> and read up on transmission line loss, dB calculations, etc.
>
> My interest has come up as I'm planning on installing a flag pole vertical in my
> yard, due to CCRs, and the coax run will be around 100 feet outside the house,
> plus 28 feet inside.
>
> Here's my take on line loss, and please correct me if I'm wrong.  For today's
> discussion, let's assume the power is 100 watts, total run is 100 feet, that I
> can tune this vertical up to 50 MHz, and the SWR at the antenna hits 5:1 (it
> won't, as I'll have a remote tuner there, but just for discussion...).
>
> Davis RF-brand "RF-9914F Bury-Flex" has a published matched line loss of 1.1
> dB/100 ft at 50 MHz.  Per the Handbook, a 5:1 SWR at the load will add around
> 1.2 dB of loss, for a total of 2.3 dB over the run, for about 60% efficiency.
> (I'm still fuzzy on the percentage calculations.)
>
> A variety of brands of RG-8X have a matched line loss of 2.0 dB at 50 MHz per
> 100 ft.  A 5:1 SWR adds 1.7 dB, for a total of 3.7 dB loss, for a little over
> 42% efficiency.
>
> Am I reading something wrong here, or is the 1.4 dB difference between the two
> cables correct?  Can you extrapolate that to an S-meter at 6 dB/S-unit?  If so,
> I'd say the 1.4 dB would be barely noticeable on the receiving end at best, and
> the extra cost of lower loss coax isn't worth the money.  Operation at HF would
> be even less of an difference than at 50 MHz.
>
> I realize there may be other considerations as to the type of coax used that
> could sway one's decision.  In my case, I want direct buriable, and I haven't
> come across any 8X that allows this.  Longer runs and higher SWR would obviously
> sway the argument as well.
>
> RG-6 seems to be in between the RG-8X and Bury-Flex as far as loss, but the cost
> may be much better (from what I've read here).
>
> In my searching, I came across a pretty handy document from Cisco relating to dB
> loss and gain:
> http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a00800e90fe.shtml
> Thanks for your input.
>
> 73,
>
> Mike
> KW1ND
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>
>
>  

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[Bill VanAlstyne W5WVO]

[hidden email] wrote:

> Third is maxing out performance. While an S-unit of loss will
> normally not be noticeable most of the time, when things get tough it
> can be the difference between a QSO and no QSO. This is one big
> reason the big guns work stations the rest of us don't.

Right on, Jim. This is a very important point. What's the difference between a
signal just barely above the noise floor that you can copy and a signal just
below the noise floor that you can't copy?

(A) About a dB or dB-and-a-half, and
(B) A new country/grid/zone/state, or not.

That dB wouldn't be even measurable if the guy's signals were S9 and 50 dB
above the noise -- but when you get down to the bleeding edge in weak-signal
work, that last dB or dB-and-a-half is what separates the big guns from the
wanna-be's.   :-)  There's the difference in your coax right there.

> Fourth is purely esthetic - some folks like knowing that they have the
> highest efficiency reasonably possible.

What's the difference in cost, twenty or thirty bucks? Come on, you spend that
much for dinner with the XYL, and by the next day, it's all down the drain.
Literally.  :-)  Your coax will be there helping (or hurting) you for years.
Get the best stuff you can. You'll never regret it.

Bill W5WVO

>
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