Proper Coax Lengths

(1/3) > >>

Kent Williams:
Howdy all.

I was recently reminded of something in my youth in regards to coax transmission line lengths.  Back in the late 1950s when I was studying for my General Class license I remember a chart in the RADIO AMATEURS HANDBOOK that showed what coax lengths NOT to use.

I was using a 1 foot Radio shack jumper in the shack between the rig and the transmatch that jumper began to break down after 20+ years of use.  I had a 12 foot length of RG-8X lying around I thought it might work better.  Not so, the SWR jumped from 1.1:1 to 1.6:1!!  I put the old 1 foot jumper back in place and the SWR again dropped to 1.1:1.

I have lost my handbook as well as my ARRL ANTENNA HANDBOOK in the many moves I have made throughout my career and the band that I will be using starting this week QRP was not available back then. (30M)

So, I have been trying to figure it out myself.  As I remember, one-half wave reproduces while one-quarter wave coax lengths transform.  I also remember velocity factor has quite an effect with RG-8 / 58 length being about .66.

Presuming I am correct that 1/2 wave lengths reproduce, then 468/10.1 x velocity factor should give me an electrical 1/2 wave length of coax.  Since I have just recently switched to Bury-Flex with a velocity factor of .82 the formula would be: 468 / 10.1 x .82 = 37.996 feet.  My coax was ordered with connectors installed and was only available in 25 foot increments.  Due to my antenna / shack location I ordered a 75 foot length.

So, it would seem that a good length would be 2 x 37.996 feet or 75.992 feet.  That is pretty close to 75 feet.

Bottom line, am I correct or way out in left field on this?

The initial example of SWR change was on 14.050 MHz, but since there were four different pieces (types) of coax involved it would be really tough to figure it out, at least for me, but here it is:
Freq:     14.050 (1.1:1 SWR)
Coax 1:   75 feet Bury Flex
Coax 2:   1 foot RG-174 (Window feed through)
Coax 3:   6 feet RG-8X
Coax 4:   1 foot RG-58

Freq:    14. 050 (1.6:1 SWR)
Coax 1:    75 feet Bury Flex
Coax 2:    1 foot RG-174 (Window feed through)
Coax 3:    6 Foot RG-8X
Coax 4:    12 Foot RG-8X

If 75 feet of Bury Flex is the way to go, then I will bite the bullet and drill through the floor and not use the RG-174 and RG-8X and RG-58 jumpers since the Argonaut has built in SRW reading capability and I can take the transmatch out of the line.

Thanks in advance,
Kent (K8QLW)

Dale Hunt:
The coax length really shouldn't matter.  If the SWR is
low there is no reason to choose a half wave over a
quarter wave of feedline:  the SWR will still be the same
at the radio end (neglecting the additional loss in the
longer cable.)

There are some situations where changing the length of
the coax will change the SWR, but they have nothing to do
with the coax itself.  Generally this is when one uses a
dipole without a balun, or other antenna that tends to
be susceptable to RF on the outside of the coax.  (Some
copper pipe J-pole designs are notorious for this.)  In
that case, the coax shield is part of the antenna, so
varying the length changes the SWR because you are actually
changing the antenna dimensions.

You can't really tie this to quarter or half wave line
lengths, either, unless you know how the rig is grounded
and what else is connected to it.  If the rig is ungrounded
(a QRP rig sitting on a picnic table, for example, with
a very short battery cable) then a quarter wave feedline
would be the worst situation, but the same rig with a
good ground connected immediately to it would have the
greatest RF on the coax when it is a half wavelength long.

The coax length is important when you want it to perform
a specific impedance transformation (or lack of
transformation), such as a quarter wave of 75 ohm coax
to match a 112 ohm loop to 50 ohms.  But if you are using
a 50 ohm meter and 50 ohm coax, the length of the coax
won't change the SWR in the shack unless it actually
changes the SWR at the antenna end.

Bob Lewis:
IF you have an SWR other than 1:1 then changing the length will change the impedance at the transmitter (but not the SWR on the line). A half wave (or multiple) feed line will repeat the antenna impedance at the transmitter. A 1/4 wave will cause it to change the most. If you have a reasonable match between the feed line and the antenna then the best length is whatever reaches.

If your SWR reading is changing with small changes in feed line length then you have RF currents flowing on the outside of the coax shield. Large length changes will also change the feed line loss and that will cause the SWR reading at the transmitter to be less with a longer line. If the line length is infinity then the SWR at the transmitter will always be 1:1 regardless of what you put out at the antenna end - but you won't radiate much power because the loss is also infinity :-)

Dale Hunt:
The variation in impedance at the rig (even when the SWR
is constant) might make a difference in some cases.

Some rigs are more tolerant of higher impedance loads than
lower ones.  For example, with a 1.5 : 1 SWR on 50 ohm
coax the impedance at the rig could be 75 ohms or 37.5
ohms (or somewhere in between with some reactance.)  My
old Ten-Tec rig was rated for "50 - 75 ohm" loads, so it
would be happier if I chose a length of coax that presented
the higher impedance.  And while the old tube rigs with
pi-network outputs could match a range of impedances, they
were often limited on the low impedance end by the maximum
capacitance of the output capacitor.  I remember W6SAI
writing about an amplifier that actually couldn't quite
match 50 ohms on the bottom of 160m, so he adjusted his
tuner for 75 ohms instead, which was within the tuning
range of the amp.

For any load impedance, there will be certain line
lengths that will give a higher impedance, and adding
or subtracting a quarter wave from that will give the
low impedance for that SWR.  But you can't generalize
this, because it depends on the exact load impedance
and not just the SWR on the line.  If you know what the
impedance is, however, you can choose a line length
accordingly.  This is one reason why there was a recommended
coax feedline length for the G5RV:  it has a low input
impedance on 80m, which was below the range that some
tube rigs could match.  (A typical matching range might
be 40 to 300 ohms.)  By specifying a quarter wave
transmission line (adjusted a bit for the desired 80m
frequency range) this impedance was easier to tune by
those rigs.  But if the impedance had been on the high
side, that feedline length would have been a poor choice.

So if you know the impedance at the antenna and the
range of impedances that your rig (or tuner) can match
most efficiently, you can choose a length of coax that
will provide a favorable load impedance.  But without
knowing that, there is no general "good" or "bad" length
for feedlines.

Kent Williams:
Complicated subject indeed.

Well, I bit the bullet and, after 3+ hours of drilling through the floor of our double wide, pulling up skirting, clearing all spider webs out of my way and keeping an eye for snakes, I attached the end of the Bury Flex to an arrow shaft that I had poked through the insulation.  My 11 year old grand daughter, bless her heart, worked with me to pull up the coax into the shack.  That left me with just 75 feet of Bury Flex and no jumpers.

I got everything hooked up, said a prayer and keyed up the Argonaut.  The reflected SWR reading was almost none existent.  I am one happy camper. PTL!



[0] Message Index

[#] Next page