Hi Mike,
The following is assuming that you would not knowingly throw away dB on
suboptimal 160 antenna construction, provided the issues were manageable in
your situation. I have clear evidence, for years now, that a local 160 QRP
operator, with the following issues properly dealt with, outscores and has a
louder signal than maybe half the 100 watt 160m signals on the air. I have
challenged some of the latter, and from some of them encountered a defensive
retort approximately translated "I worked so-and-so DX, so my XXXX antenna
is fine, thank you." That equates probably to the fact that so-and-so DX
had VERY good DXpedition operators and a superior listening setup and THEY
made up for the lost dB's that MUST be in some of these transmitting setups.
So in interest of those getting in the water for the first time, the
thumbnail non-technical version of the nasty math, model and measuring
experience follows:
You didn't mention whether a tower was one of the supports. Do this antenna
in the trees if you can. If you take the L up next to a tower, even spaced
10 or 15 feet, you are actually hard-loading the tower, and you would be
better off dealing with loading the tower directly. There are many
tower-supports-L situations where models show that there is more current in
the tower than the L. Consider the two connected directly by a transformer
with a step DOWN ratio from the L to the tower. If the tower does not have
a dense radial field out from its base, it will be lossy, EVEN IF you have
an excellent elevated radial system from the L. You will be FORCING current
from the base of the tower into the dirt, and along every tower conductor,
presumably buried or laying on the dirt. It's a PITA to model, but the
results are almost always bad in the model and careful culling of tales from
the field confirm it. You are throwing away dB's you could otherwise use to
get to that new country that's close to the noise in your RX.
I've gotten tower/dirt combo's in models to kill 4 or 5 dB, and models tend
to UNDERESTIMATE dirt loss. Beside the tower base to dirt loss, there is
the problem that no matter whether radials are from the tower, or from the
L, now the radial current can no longer be opposite phase and current equal
to the COMBINATION of the tower and vertical currents, which is a complex
addition of unequal phase and unequal amplitude. The radials will have the
feedline phase and amplitude. Now even more current is induced in lossy
tower vicinity dirt.
If you have a tower on the property, put your 160 L as far away from it as
possible, and if you MUST use the tower as one support because that's all
you can do, then use the tower to support the far end of the horizontal, to
minimize forced current in the tower. ** Do not use the tower to support
the vertical run. **
But read on if vertical run is not supported by a tower.
If you can get up 12 elevated radials, then try to get up 16 to 20 instead.
That will give you a "near dense radials" rating, good to the point of
diminishing returns. If you can do 30, then do it, getting you that last dB
shard transmitting. Make sure that the radials are 1) all the same height
and 2) same length, and 3), IMPORTANTLY, that they are equally spaced around
the compass. You are constructing a device that is creating a counter-field
to the RF field. The field from the vertical radiator is already smoothly
distributed at the dirt. The 16-20 is to get the field from the radials
smoothly distributed at the dirt, opposite phased, and smoothly and more
totally cancel out. 30 is to get that cancellation out past diminishing
returns.
After that, there are a number of good multi-impedance Ununs that will have
28, 22 and 16 ohm matching configurations or something like that and will
deliver a good match. I assume you are going to trim the horizontal part of
the L for resonance. Then just use the Unun connectors that deliver the
best SWR. Then immediately use a common mode current block rated at 160
meters on the shack side of the Unun, and then ground the shield of the feed
coax on the shack side of the common mode current block, about 20 to 25 feet
away from the antenna. Do not ground the antenna on the antenna side of the
current block. You alread have DC grounding through the Unum, common mode
block, coax shield, and shield ground. This will isolate the antenna from
feed line common mode noise, AND keep the feedline shield from accepting
counterpoise directed antenna current. You want the energy on the elevated
radial wires to be equal to the energy on the vertical wire, at the
feedpoint. If an unblocked feedline just happened to show a low Z, it can
siphon off energy from the radials. Now the field from the radial current
and the opposite field from the vertical radiator's current are NOT equal,
and the field is is only partially cancelled, and you have lossy induced
current in the dirt underneath.
Why NOT start off with everything done right? Add an amp and some RX
antennas and go kill some pileups. Or go QRP and embarrass people. :>)
73, Guy.
On Mon, Sep 5, 2011 at 8:01 PM, Michael G. Carper <mike@wa9pie.net> wrote:
> Okay... great responses all. Here's what I think I'm gonna do...
>
> I'll go vertical with an inverted L. I'm thinking to be resonant around
> 1.825MHz, I'll need about 128.2' of wire. I'll go up about 70'... then the
> rest will be almost exactly horizontal. I'll go with at least a dozen
> elevated radials about 134.8' long. I'll terminate those at a post where
> I'll have an insulator for the "driven element" and one where the radials
> terminate. I'll stick an SO-239 there to make things easy. I figure this
> should be resonant at the chosen frequency.
>
> Later, I'll put up at least one beverage antenna for receive (but there's
> no
> way I'll have a chance to do that initially). Does that sound reasonable
> as
> a starting position?
>
> Thanks to all who talked me out of the B&W Folded Dipole.
>
> Mike, WA9PIE
>
>
> -----Original Message-----
> From: topband-bounces@contesting.com [mailto:
> topband-bounces@contesting.com]
> On Behalf Of Michael G. Carper
> Sent: Sunday, September 04, 2011 11:06 PM
> To: topband@contesting.com
> Subject: Topband: New entry to TopBand with antenna question
>
> Hey folks.
>
>
>
> I've been a ham for 37 years, but I've spent my entire ham "career" on
> 80-10m (with the exception of 7 countries worked on 160m about ten years
> ago). For the most part, the same obstacle has kept me off TopBand that
> prevents most - antenna space.
>
>
>
> Now that I've made DXCC on all bands 80-10m (and very close on 6m). and
> since we've recently moved to a place with 7-acres and a nice tall
> tree-line, I figure my options for 160m are better. That said - we're
> leasing this place and I've probably got 2-years here.
>
>
>
> My goal is simple - make DXCC on 160m within the next 2 years.
>
>
>
> So I'm looking for some advice about the antenna.
>
>
>
> I'm very tempted to buy a B&W folded dipole and call it done. but I'm
> curious about what others are doing. I've got an amplifier, but no
> high-power tuner. So I want to avoid a tuner - the folded dipole seems to
> get me there and I can put it up at 70' or so in the elm trees.
>
>
>
> Thoughts?
>
>
>
> Mike, WA9PIE
>
>
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
> _______________________________________________
> UR RST IS ... ... ..9 QSB QSB - hw? BK
>
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
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