Topband: Ground screen ????
Joshua M. Arritt
jarritt at vt.edu
Wed Mar 2 11:02:46 EST 2016
Galvanized Fe material is problematic from an R standpoint as well (vs.
Cu). This results in a "lossy" ground plane. That condition has real
impact the effectiveness of the antenna system, when compared to an
all-Cu (or at least a lower-R metal) plane.
Immediately perceived consequence of the poor inter-section bonds is
walking VSWR, but other factors (including rad efficiency) are also
Bonding of dissimilar metals (Cu <-> Fe) requires careful method
consideration to hold corrosion at bay. Whatever scheme is used to
connect to this ground plane should keep that in mind.
The PIM issue was rightly treated from a TX standpoint by KZ1W. However,
the PIM issue /may/ be more salient from an RX standpoint -- not
necessarily as coherent intermod from surrounding transmitters per se,
but rather as perceived as elevated RX noise. Inv.L over a
sectioned-Fe-plane-counterpoise may make a markedly worse RX antenna
than the same Inv.L over a modest, well-constructed Cu plane. The local
ambient noise environment will be a factor in the magnitude of noise
contribution from this issue (might be quiet vs. power lines, for
instance). Certianly, one will want to investigate alternative RX
I don't feel as though lightning protection is any bit a
"more-complicated" an issue here; bonding the coax with a good low-Z
method to earth before it enters the shack is an important first step.
Including good common-mode elimination at the feedpoint should increase
protection to some degree as well. There are methods beyond these steps
-- situation-dependent -- which may help further reduce risk. Risk
/reduction/ is all we can do in this department anyway....
All the problems in mind, a metal roof is better than no ground plane at
all. It's not an ideal situation by any means, but armed with the sound
knowledge presented in this thread, I would not be "above" of employing
just such a scheme if:
- I were severely limited in land space,
- had a means of separate RX aerial operation,
- a willingness to forget tuner settings often for a given F(x), and
- a BIG bug bite for low band operation!
- Josh / KF4YLM
On 01.03.2016 22:09, Douglas Ruz (CO8DM) wrote:
> Need some ideas from your side...
> I am trying to find a good solution for my 160/80m antenna.
> I am thinking about some kind of Inv L with an 80m trap as a radiator for both bands.
> But, what about the counterpoise...Unfortunately in my small city lot I can´t install radials and I can´t get parts for the FCP yet...what about a Ground screen ???...I have a galvanized roof...it is about 50 ft long and 16 ft wide...galvanized sheets are 3 by 8 ft long and all are bolted together. The roof is about 15 ft high.
> Can I use this roof as a ground screen for the 160/80m antenna ???
> Thoughts ?,
> 73....Douglas, CO8DM
> P.S:1- I finish my 50 ft tower installation.
> 2- Thanks for all QSO´s on 160m before ARRL CW contest with me and with our contest station T48K.
> Topband Reflector Archives - http://www.contesting.com/_topband
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