Ladder Line Entry to Shack
K8JRM
This is an age old question - what is the best way to get 450 Ohm ladder line
into my shack antenna tuning unit (ATU)? Here is my situation; I have two 4"
PVC tubes inserted into the exterior wall that allow the entrance of coaxial
cable into my shack. When I run the ladder line into the shack through one of
these ports, the multi-band wire antenna fed with the ladder line goes wild! I
can not get the antenna to tune with any of my ATUs.
I have received several suggestions on how I can overcome this problem. The
most promising one seems to be to use two short pieces of coax with center
conductor connected to either side of the ladder line. These pieces of coax, in
turn, would be fed through the PVC hole into the shack and the center
conductors connected to the ATU. The shield on the coax would float with no
connection to any thing. This solution sounds good; however, common sense
tells me that ladder feed line will still suffer because of its proximity to
other objects. Any thoughts on this solution?
Another possibility is using a piece of Plexiglas place under a metal framed
window pretty much as shown in the old hand books. My problem with this
solution is that the window is on the other side of the room from operating
table, the two coax entry ports and AC power.
Short of placing the tuner outside in the elements, is there a solution to my
problem? I would be interested in hear what has been tried!
Tom, K8JRM in OKC
Ian Jackson
writes
If you already have the two (presumably fairly short) pieces of coax
lines already in place, I reckon that the first thing you should try is
to ground the two shields to the case of the ATU (which I presume is
your shack safety ground, and maybe also your RF ground). This should
screen the inner conductors from other nearby conductors, and keep the
feed balanced. If you don't ground the shields, you might as well not
use coax.
Obviously, using coax will change the impedance of the line for a short
distance, and may affect the settings of the ATU (more on the higher HF
bands, of course). But with luck, the ATU will still tune OK. The
increase in feeder losses should be negligible.
An alternative is to go from the ATU and through the wall using low
impedance twin feeder. The spacing between the conductors will be much
smaller than the for the 450 ohm feeder, and it should not have much
opportunity to become very unbalanced due to the proximity of nearby
conductors. It might be advantageous to twist it. I understand that, if
the insulating material is suitable, twin mains power cable can be used
as twin feeder, especially on the lower HF bands. The current rating of
the cable will, of course, depend on what power you intend to run.
Ordinary lighting twin flex should handle a couple of hundred watts.
But, if you've got a bit of 50 or 75 ohm twin, all the better.
--
Ian
Owen Duffy
Many configurations of the type you describe are widely misunderstood.
To help you to analyse the configuration, keep in mind that for a coaxial
cable in TEM mode, the current flowing on the outside surface of the inner
conductor is accompanied by an equal magnitude but opposite phase current
flowing on the inside surface of the outer conductor.
Consider a break in the sheild as a circuit node with at least two
connections, the inside surface of the outer conductor and the outside
surface of the outer conductor.
The configuration you describe will make essentially no difference to the
common mode current, the feedline is not effectively shielded by the outer
conductors.
Owen
co_f...@yahoo.com
Tom, you confuse me. Is the ladder line in the PVC tube all by itself?
Or are there other conductors in the tube with the ladder line? If all
by itself, then there has to be some metal conductor around the PVC.
If the ladder line has to be in a PVC pipe with other wire, Stuff the
PVC pipe full of rags or crumpled newspaper to fill the PVC and raise
the ladder line as far away from the other conductors as possible. See
if that helps.
Another possibility is to put two 1/2 inch pvc pipes inside your 4
inch pipe, split the ladder line down the middle and run each wire
through a 1/2 pipe. Some splicing will be necessary, but your antenna
tuner won't care.
My antenna is a 160 meter lazy quad (horizontal) fed at a corner with
225 ft. of 600 ohm feed line. This line comes through the wall of my
mfg home (aluminum siding) and passes through a DPDT switch to 400 ohm
ladder line to the antenna tuner. The wall insulators are ancient
ceramic units made specifically for feed line through the wall. The
tuner takes care of whatever matching is needed. Works great!
Paul, KD7HB in Oregon
Sal M. Onella
"K8JRM" <k8...@cox.net> wrote in message
news:Qnyul.105282$EO2....@newsfe04.iad...
> This is an age old question - what is the best way to get 450 Ohm ladder
line
> into my shack antenna tuning unit (ATU)? Here is my situation; I have two
4"
> PVC tubes inserted into the exterior wall that allow the entrance of
coaxial
> cable into my shack. When I run the ladder line into the shack through
one of
> these ports, the multi-band wire antenna fed with the ladder line goes
wild! I
> can not get the antenna to tune with any of my ATUs.
What about enlarging the hole and covering it with a piece of plexiglas with
two bolts through it, spaced the same distance apart as the gauge of the
ladder line. Connect sections of ladder line to both ends of the bolts --
under the bolt heads on one side and under the nuts on the other.
MFJ makes something along these lines. Please see:
http://mfjenterprises.com/Product.php?productid=MFJ-4602
Finally, have you tried tubes of material other than PVC? I ran some
300-ohm TV twinlead through an empty Sue Bee honey squeezer tube and the
pictures were fine. (This was before I owned a signal meter, so I don't
know if it had any bad effect.)
"Sal"
Ron Walters
Tom,
Sounds like your running the ladder line with some other conductors in
the PVC.
You never said how far the entry point was to the ATU, if its relatively
short less than 15' or so I would recommend you run a piece of coax from
the ATU through the PVC to a 1:1 balum located outside. there would be
some loss in the coax/balum but it would be low on 160-30M and some what
higher on 20-10 meters. I don not think you can quantify the amount of
loss but I don't think you will see it via RST reports. I have
successfully used this method over several years since I did not have
the 4" PVC that you luckily have for the coax and other conductors.
The above suggestion is made on the assumption that the ATU has a
unbalanced output for coax and is not a balanced only tuner.
With respect to the addition of two (2) pieces of coax as a feed through
on the inside of the home the shields are connected together and then
joined to the ground system in the shack. From the feed through point
then you can continue with the ladder line to the ATU.
I recommend the remote balum since it helps cut down on any coupling of
RF to other conductors in the PVC. Use a good current type balum since
the swr is likely to vary over the multiple band use. The length of
ladder line from the balum to the feed point of the antenna is some what
critical and there is lots of articles on the subject on the WEB, I
believe DX Engineering has some excellent white papers on this subject
for remote balums.
73 de
Ron W4LDE
Richard Clark
>I can not get the antenna to tune with any of my ATUs.
Hi Tom,
You have an unbalanced load whose common mode current is heavily
influenced by the proximity of conductors in this case. Previous to
this, the load was still unbalanced, but you didn't notice it as much.
Solve the balance and keep the line away from nearby conductors.
73's
Richard Clark, KB7QHC
Bob Miller
>Hi Guys,
>
>
>This is an age old question - what is the best way to get 450 Ohm ladder line
>into my shack antenna tuning unit (ATU)? Here is my situation; I have two 4"
>PVC tubes inserted into the exterior wall that allow the entrance of coaxial
>cable into my shack. When I run the ladder line into the shack through one of
>these ports, the multi-band wire antenna fed with the ladder line goes wild! I
>can not get the antenna to tune with any of my ATUs.
I use the MFJ feedthrough board # 4602. It has two ceramic feedthrough
insulators for balanced line, with other feedthroughs spread across
the board for coax lines, a random wire and a ground wire. MFJ now
makes a whole line of these boards, and they come complete with
weather stripping and burgler bar. No problem tuning the balanced
450-ohm line, coming in to the balanced posts on an MFJ tuner.
The whole setup also makes it very easy to disconnect antennas from
the outside when it gets stormy.
bob
k5qwg
Peter
The two short pieces of coax with center conductor connected to either side
of the ladder line is perhaps the best solution. It was covered in QST/The
Doctor is In, June 2008. In the article it suggests using the best quality
coax on hand, keeping the run to the absolute minimum and the coax shields
should be tired at both ends and connected to the station RF earth. This
configuration introduces a small section 100ohm nominal impedance
transmission line into the antenna system which should have minimal losses,
but most important is that it is still part of the balance transmission
line. The losses for say RG213 at 4mtr would be almost nothing on 80mtr and
perhaps up to 2db on 10mtr.
Obviously the lengths of coax need to be exactly the same length.
If you would like a copy of the article drop me email.
Peter VK6YSF
http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm
co_f...@yahoo.com
Hi, Peter.
I read the QST article and thought it was crazy and so I tried
replacing the 5 ft length of ladder line from my antenna tuner up the
wall to the two feed through insulators going through the wall to the
600 ohm feed line outside.
With the 5 ft ladder line I could tune the antenna, a 160 meter lazy
quad loop, to all bands through 15 meters. With the 5 ft section of
double coax, just like the doctor said, the antenna would not tune for
20 meters or any higher band. I put the old ladder line section back.
Remember what all the discussions say about keeping the feed line away
from metal objects? Here we are introducing a 5 ft length of metal
just 1/4 inch from the feed line. The extra capacitance just kills the
feed line. Yes, I was using RG-213.
So, the doctor in nuts, as far as I can tell.
Paul, KD7HB
Peter
Paul, KD7HB
Paul
That's interesting and noted as I was intending to use that approach to get
the balanced line into my new shack, which is a metal shed. I guess it may
be back the old drawing board. Any suggestions?
Peter VK6YSF
http://members.optushome.com.au/vk6ysf/vk6ysf/main.htm
Owen Duffy
news:7dfa05c5-dc4a-4962...@v1g2000prd.googlegroups.com:
...
> Remember what all the discussions say about keeping the feed line away
> from metal objects? Here we are introducing a 5 ft length of metal
> just 1/4 inch from the feed line. The extra capacitance just kills the
> feed line. Yes, I was using RG-213.
Paul,
The "5 ft length of metal just 1/4 inch from the feed line" is not a good
method of analysing the behaviour.
The treatment of the shield ends is very important to the operation of
these things.
If the shield ends at each end are tied together, and each end
effectively grounded (and that is a big ask), then any common mode
current on the incoming ladder line flows to ground via the outboard
ground connection, and the section of dual coax transforms the impedance
seen looking into the ladder line in a quite predictable way, and with
some loss. The transformed impedance might not be within the range of
your tuner.
If the outboard shield ends are not effectively grounded, then the
configuration does essentially nothing to reduce common mode current
entering the shack, so the system is a fails it its primary objective.
Owen
Owen Duffy
news:zYedndKerNTQySHU...@westnet.com.au:
...
> The two short pieces of coax with center conductor connected to either
> side of the ladder line is perhaps the best solution. It was covered
> in QST/The Doctor is In, June 2008. In the article it suggests using
Thanks Peter, I have had a quick look at the article.
The "Doctor" describes the configuration in his Figure 4. I have posted a
copy of the figure to assist discussion, it is at
http://www.vk1od.net/lost/ShieldedTwin.png .
The treatment of the shield ends is important detail in understanding how
this works.
If the ground connection on the antenna side of the shielded line has a
very low impedance to ground, almost all common mode current on the open
wire line will flow to ground via that connection, and very little common
mode current will flow into the shack.
For example, lets consider a G5RV at that the transition to shielded twin
is at the bottom of the so-called "matching" section. Using a coax pair
was comprised 5m of RG213, the impedance looking into the open wire line
on 80m would be somewhere around 10-j340.
The transformation and loss in the twin RG213 can be calculated using
TLLC (http://www.vk1od.net/calc/tl/tllc.php) with a load of 5-j170 on
RG213. The result is 1.9dB of loss in the 5m of cable, 35% of the power
into the twin RG213 is lost as heat. The impedance looking into the twin
RG213 would be 2-j80, a very challenging load for efficient
transformation to 50 ohms.
Now, this is a pretty awful case, and they won't always be this bad,
though some will be worse. This shows a method of analysing the
configuration.
The "Doctor" says in his article "Whilst it is critical for proper
operation for the shields of the two coaxes be tied together at each end,
whether they are tied to "ground" is not important for operation, but can
make a big improvement in lightning protection."
I disagree with the last statement, the treatment of the ends of the
shields wrt ground is very important to analysis, the configuration
behaves differently for different ground configurations. For example, if
the antenna end of the shields is left disconnected, the common mode
current on each side of the antenna end of the shielded twin is
essentially equal and common mode current flows on the outside of the
shielded twin. The shielding does not eliminate external fields from
common mode current.
Owen