Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1]   Go Down
  Print  
Author Topic: Short line, ladder or is coax ok?  (Read 1449 times)
KD0VEY
Member

Posts: 22




Ignore
« on: May 06, 2013, 06:13:31 AM »

I have searched and found similar things but no answer to my question. Any help would be appreciated.
I am making a 160-10m dipole, and using an antenna tuner.
My feedline only needs to be 25 feet. I can probably get it a little shorter.
My cave is in an all metal building. There is no way to get out without contacting the metal siding or roof. I cannot cut a hole through the metal large enough to keep the ladder line away from the metal. The feedline would need to exit in the space between the metal roof and wall. The feedline would be within 6 inches of the metal for about 6-12 inches. The feedpoint would be several feet from the metal wall.
1) Should I be using ladder line? Or, since it is only 25 feet would coax be ok?
2) I saw on the net where someone suggested using 2 pieces of coax as a ladder line when going through metal. The shield would be connected. Would there be any benefit to this rather than just using 1 line of coax for this distance?
3) With the metal surroundings, do you think I may get more rf exposure from ladder line running 4 feet above my head?
Thanks for any help.
Logged
AA4HA
Member

Posts: 1418




Ignore
« Reply #1 on: May 06, 2013, 06:30:07 AM »

If you wanted to go through the wall of the building you could just use two feed-through bushings, spaced the same distance apart as the ladder line. Then just wire the ladder line to the outside posts on the two bushings and another piece of ladder line wired to the inside of the bushings and down to your tuner/radio.

If you are terminating into your radio or antenna tuner with coax you could even put the balun to convert to coaxial right outside the wall and run in with a short piece of coax.

Since it is a metal walled building you could use a SO-239 double female bulkhead connector for the wall penetration. This gives you an additional benefit of grounding the coax shield to the wall.
Logged

Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
WB6BYU
Member

Posts: 13225




Ignore
« Reply #2 on: May 06, 2013, 09:45:05 AM »

It isn't hard to lose 1/2 to 3/4 of your power in a 25' length of coax when operated
at a high SWR (as would be the case in your situation on at least some bands.)
You may be able to adjust the antenna length to reduce the loss on certain bands,
but not over the whole range.  Running open wire line close to metal for a short
distance isn't anywhere near as lossy.

The most common side effect of running balanced line near metal is a shift in the
characteristic impedance over a short distance.  This can be an issue if the line is
matched (such as a 300 ohm line feeding a folded dipole) but we rarely use matched
lines like that in general ham use:  you're going to be using a tuner anyway, and
it will correct for any minor changes in load impedance due to the metal.

You still need to provide enough insulation that you don't short the feedline, of
course.  At one point I had an old sock wrapped around a piece of twinlead
where it went out through a metal window frame (though I'll admit that wasn't
ideal.)  But if you can arrange a couple of plastic plates on either side of the
feedline to prevent physical damage as well as arcing if it happens to be a high
voltage point, that may be enough to get you on the air.

Otherwise you can cut two very short pieces of coax (a foot or so at most) and
pass them through the opening, then connect the balanced feedline to the
inner conductors.  Or insert a pair of coax feed-through fittings into the gap and
connect the feedline to the center pins with banana plugs.  Either will reduce the
length of coax and reduce the potential losses.
Logged
K4RVN
Member

Posts: 767




Ignore
« Reply #3 on: May 06, 2013, 12:07:48 PM »

Could you give a little more info on the design of your antenna and the tuner info? I would vote for the feed through coax fittings installed in the metal building wall spaced to match your ladder line.
Then extend the shortest possible ladder line to your tuner balanced line terminals. You can use pins or solder to make suitable connections on the ladder line ends for the feed through as stated before.
The coax feed would probably work with your so 239 fitting on your tuner but not as well as the ladder line feed. At any rate ,depending on your antenna design, it may be touchy to tune on some bands. I would also buy me an inexpensive field strength meter if concerned about the RF exposure from the 4 ft. above your head. No guessing that way. Both Tisha and WB6BYU know more than I do about tuners no doubt in my mind. I do not remember if you mentioned how much power that you would run. That is a consideration for RF as you know.
« Last Edit: May 06, 2013, 12:10:04 PM by K4RVN » Logged
WB6BYU
Member

Posts: 13225




Ignore
« Reply #4 on: May 06, 2013, 02:01:20 PM »

RF radiation from balanced line is often overrated.  To minimize the radiation use
feedline with the wires closer together (such as 300 ohm twinlead), keep the line
balanced with respect to metal items, and either gently twist the line or arrange it
so where you sit is equidistant from each conductor. 

But it isn't uncommon to have more radiation from a piece of coax connected to a
dipole (due to common mode current) than from the same length of balanced line.
Logged
K4RVN
Member

Posts: 767




Ignore
« Reply #5 on: May 06, 2013, 04:35:16 PM »

I forgot to mention that if your multi-band dipole is a fan dipole, forget the  ladder line and run coax all the way. You can still use your tuner with coax.  Then install a MFJ or other 1 to 1 choke/balun to prevent RF on the coax up near the antenna.  I also use an MFJ 915 line isolator at the transceiver on my  antenna output. My field strength meter indicates no stray RF in the shack.

Frank
« Last Edit: May 06, 2013, 04:37:35 PM by K4RVN » Logged
K3VV
Member

Posts: 13




Ignore
« Reply #6 on: May 06, 2013, 04:54:24 PM »

There are several feedline loss calculators out there.  Here's one:

http://www.arrg.us/pages/Loss-Calc.htm

Set up the following calculation:  f=30MHz, SWR=20:1, Length=25 ft   This represents a pretty bad match at a pretty lossy frequency.  If you think you're going to have more or less of a mismatch than that, adjust the SWR accordingly. 

Compare the loss difference between a good coax like LMR-400 or Belden 9913 and that of your ladder line over a 25-foot span.  Do the same over a 10-foot span to compare the losses to see what happens if you run coax only from the exterior wall to the station.

If it rains, snows or ices where you live, and if the outdor part of the ladder line is exposed to the weather, sure to run the worst-case calculation for wet/icy ladder line as well as for dry.  The calculator offers both options.

A half-hour with the calculator should make the decision pretty obvious, I think.

Logged
K0ZN
Member

Posts: 1546




Ignore
« Reply #7 on: May 06, 2013, 06:50:57 PM »

 A pretty good rule of thumb is to try the SIMPLE fix first.  i.e.  I would try ladderline. If you can keep it a true 6 inches from the metal roof I highly suspect it
 it won't be a problem at all. If it only runs parallel to the roof for 6 to 12" in the worst case, that is kind of a non-event. Problem is, when you start getting
 more complex by using coax, baluns, etc. those items can/may introduce some pretty significant loss, especially if operating at high SWR.
 Balun's can get real weird at high SWR and higher powers. I don't know how much power you plan to run, but with
 coax at high SWR you run into a realistic chance of voltage break down. The hard fact is coax is just not good stuff to use at high SWR.
 .....and trying to feed a 160 M dipole on multiple band absolutely guarantees some very high SWR on some bands.

 The suggestion to use feed through insulators spaced the distance of the line is a good one too..... if you can drill the wall and it is thin, like sheet metal.

 If it was me, I sure would try ladderline if it could be spaced 6" off the roof and the run by the roof was only 6 to 12". Keep in mind that any coupling to the roof, etc. will very, very minimal on bands from 20 M on down.... probably "negligible" on 40M and lower. My gut feeling is it will work fine on all HF bands (in terms of interaction with the roof, etc.)


73,  K0ZN
« Last Edit: May 06, 2013, 06:53:33 PM by K0ZN » Logged
WB4SPT
Member

Posts: 141




Ignore
« Reply #8 on: May 07, 2013, 09:13:46 AM »

If you wanted to go through the wall of the building you could just use two feed-through bushings, spaced the same distance apart as the ladder line. Then just wire the ladder line to the outside posts on the two bushings and another piece of ladder line wired to the inside of the bushings and down to your tuner/radio.

If you are terminating into your radio or antenna tuner with coax you could even put the balun to convert to coaxial right outside the wall and run in with a short piece of coax.

Since it is a metal walled building you could use a SO-239 double female bulkhead connector for the wall penetration. This gives you an additional benefit of grounding the coax shield to the wall.

I wanna say that boring two holes in metal and running open wire thru those holes, even if the holes are big, with standoffs, is a bad idea.  Something about a one turn transformer, killing the signal.  But, I haven't tried that exact experiment.
Logged
Pages: [1]   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!