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Author Topic: Help required please with Antenna installation feasibility !  (Read 522 times)
G7IVJ
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Posts: 82




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« on: January 09, 2013, 08:35:15 AM »


Gents,

Thanks in advance for your help and 73 from COLD London !

There is a piece of land that I am considering leasing from the owners to install some HF antennas, both wire antenna(s) and a vertical with radials.

The piece of land is 273 foot on each side and is pretty much square. The piece of land is 204 foot from the shack to the closest corner of the land in a straight line, and that is not considering the upright lengths to get the coax / feeder into the shack and up to the mast / support, so add on another 50 foot so the overall feeder length would be 254 foot long.

My first concern, is with the 254 foot length feeder what the losses are ? On top band, 80m, 40m and 20m using RG213 and westflex / open line feeder.

I want to install on this land:

A top band 160m dipole / doublet
80m dipole / doublet
40m dipole / doublet
20m diple / doublet

... or I could install a top band doublet fed with feeder and a balun to make a multi-band doublet. Cost wise I would prefer this option of course, as long as it doesnt lose to much performanceover having cut dipoles for each band ! So a question from this. What sort of performance difference would there be comparing a 160m multi-band wire aerial to a 160m cut dipole installed at the same height ? And same for 80m, 40m and 20m. If the differences are not too great, then a multiband wire it is for cost reasons. I could however, start with a 160m dipole, and a 80m dipole and add 40m later... But thats more fedder, more wire, no visible items in the air for people to see....

Also I want to install an 80m multiband vertical with radials. The vertical at best, would need to be installed in the centre of the field to get it away from the RF noisey buidings on the perimeter of the land, so would add an extra 120 foot or so of feeder so 254 plus 120foot = 374 foot feeder to the 80m vertical. What sort of losses would this incur ? If used westflex 103 and RG213. I could install the vertical closer to the shack, but it would also be closer to the buldings around it which are offices - however if 374 foot of feeder to the vertical is too long I could lose 100 foot and install the vertical 100foot closer to the shack

Someone local to me said they would install a top band full wave loop and feed with open wire feeder with a balun to make it multi-band. As the land is 273 foot square, I could install a much bigger than 160m full wavelength loop and use it as a multi-band antenna.

I do not have to lease all of the land, I could lease just a section of it.

Please give me some ideas and tell me what you would do ?

Looking forward to your replys, and thanks for the help

73 - Simon
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K5LXP
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Posts: 4474


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« Reply #1 on: January 09, 2013, 08:47:41 AM »

Research the patterns you get with dipoles vs multiband doublets at the height you intend to install them at.  Either one can be matched and fed efficiently but if you're putting the energy where it's not effective, the efficiency is moot.

Consider too other wire antennas and arrays depending on your goals.  Choose the antenna(s) based on pattern first, then matching and feeding them becomes an academic exercise.


Mark K5LXP
Albuquerque, NM
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WB6BYU
Member

Posts: 13149




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« Reply #2 on: January 09, 2013, 09:20:10 AM »

You can calculate the feedline losses using VK1OD's handy calculator:

http://vk1od.net/calc/tl/tllc.php

With 100m of RG-213 on 40m the loss is 1.7dB, and 2/3 of your power
reaches the antenna (assuming a low SWR.)  Losses are lower on the
lower frequencies:  on Top Band the loss is less than 1dB.   That's
probably good enough, but there are other sorts of coax with even
lower loss.  (I have a suitable piece of RG-17 out in the barn if you
want to come over and get it, but it takes two men to move it.)


I've used large horizontal loops of various types, with generally good
results.  But as Mark says, you have to look at the radiation patterns
and see how they meet your needs.  A full wave 80m loop fed in
one corner, for example, tends to radiate off the corners on 40m, 20m
and 15m, with nulls off the sides.  On 10m it has 8 relatively narrow
lobes.  it also depends how high you can install it, and the expense of
4 support masts (unless you can hang it by ropes from the surrounding
buildings.)

By contrast, an inverted vee only requires one tall support, or 2 or 3 for
a dipole/doublet.

The radiation pattern on 160m will be about the same for all options, but
on higher bands there will be a lot of variation, with various lobes and
nulls.  (This also applies in the vertical pattern, so some idea of the
distances you want to cover and the required radiation angles to do so
will also come in handy.)

Multiple dipoles (cut for each band) on a common feedpoint, or a trap
antenna, is probably best if you are using an inverted vee, as it keeps
maximum radiation from the highest point in the antenna.  Multiband
performance drops off as the slope of the wire increases.


Another option to consider is relay switching at the base to change the
antenna arrangement.  For example, if you use an 80m doublet fed with
a quarter wave of parallel conductor line down to ground level, you can
feed it in normal doublet mode on 80m and higher (it might not work as
well on 160m due to the short length), or connect both feedline wires
in parallel and feed it as a vertical against ground, which would be
effective on 80m and 160m (and likely on 40m as well.)
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