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Author Topic: Antenna decisions  (Read 3339 times)

Posts: 90

« on: July 13, 2006, 07:40:06 PM »

Hey all, I am building a station and am liminted on what I am able to use for an antenna. So far I have a Yaesu FT-857 and a FC-40. The building where this will be going is an all metal flat roof building that is 144' long by 54'. The problem is that the antenna needs to be as stealth as possible, so here a few questions.
1) Can I extend several wires from the FC-40 ( like half of a fan dipole ) so that it will tune 160 through 6 meters?

2) What would happen (radiation pattern, impediance )if I used some of the tall electric fence insulators around the parimeter of the building?

3)Can I connect the ground lug on the FC-40 to the building? How about connecting it to the antenna as a loop?

4) Any better ideas?

I am very new to HF so any held is appriciated.

Herb    KC5HMC

Posts: 5639

« Reply #1 on: July 14, 2006, 06:41:57 AM »

A much better solution would be to make sure the roof panels are electrically connected together and install a vertical in the middle of the roof. You have an excellent, built-in ground plane there.

My choice would be a Butternut HF6V or HF9V, but the Hustler series is good as well, and is cheaper.

If you put any wire antenna around or on that metal roof, the net effect will be same as placing it at ground level. You would need to get it at least 25-30 feet above the roof before the roof stopped detuning the antenna.

I would not try to load up the roof as the antenna

Lon - W3LK
Baltimore, Maryland

A smoking section in a restaurant makes as much sense as a peeing section in a swimming pool.

Posts: 90

« Reply #2 on: July 14, 2006, 07:29:31 AM »

I agree, a verticle would be my first choise, but I am restricted to a wire antenna. Great idea on electrically bonding the roof and using it as a ground plane.

Posts: 21764

« Reply #3 on: July 14, 2006, 10:10:22 AM »

What is the building (function, ownership)?

How tall is the building?

Does it have any A/C units, chimneys, vent pipes or other stuff mounted on or protruding through the roof?  If so, what are they and how tall are they?

Posts: 1146

« Reply #4 on: July 15, 2006, 08:16:52 AM »

Why not load up the roof itself and try that as an antenna?

Posts: 90

« Reply #5 on: July 15, 2006, 12:34:45 PM »

The building belongs to the company that I work for ( the station has been approved ) and is where I work. The building is our radio repair/install shop. I know, but I am used to 50 Mc up to 900 Mc. HF is a whole new ballgame to me.
I haven't been able to get a true height of the building or get on the roof to see what is up there.
The roof, front, back and one side are all metal so it would make a nice ground plane. I also thought about a 110" whip connected to the FC-40 which would allow 7 to 54 Mc. With that in the middle of the of the roof I would think that it would make a great omni directional radiation pattern.

Posts: 90

« Reply #6 on: July 17, 2006, 11:18:44 AM »

I got some picttures of the top of the building and it looks like a plain flat roof with a rubber top or what ever they use covering the metal. No AC unit on top and I didn't see any pipes sticking up ( there should be for the bathroom ).

Posts: 21764

« Reply #7 on: July 17, 2006, 03:46:35 PM »

The metal roof does no good if you can't get a very good, zero-impedance electrical connection to it.  If it's coated with weatherproofing foam, which is what it sounds like, you can't cut through that stuff.  How are you going to make the attachment to the metal?

A 110" whip with the auto-tuner doesn't make an ideal vertical antenna for 7-54 MHz.  It makes a good vertical antenna for 10m and 12m, a terrible one for 6m (54 MHz) and a *usable* antenna for 15m, 17m and 20m.  The tuner might get it to load up on 7 MHz, but it's not going to work well there.  Reason is, it's so short and has very low radiation resistance and a whole lot of capacitive reactance.  

To tune that out, the tuner will introduce a whole lot of inductance in series with it (in attempt to resonate the antenna), and this is the most lossy situation you'll ever encounter with a tuner, because it's the inductors in tuners that introduce the most loss.

On six meters, 110" is a bad length because it's a half-wave and very difficult to match.  The auto-tuner may match it, but my guess is probably not.  A 54" whip would be a much better choice.  Yes, it is possible for a whip to be *too long*, and this is one of those cases.

I'd want to get up on the roof, and walk around it to take a first-hand survey of exactly what's up there.  You're right, there probably are vent pipes.  And I'd guess there might be more stuff than that.  If the building has a furnace for heat (not electrical), there should be a very large exhaust stack for that.  If it has a gas fired hot water heater, there should be an exhaust stack for that.  If it has air conditioning, it's common to place the outdoor exchanger portion on the roof, when a roof is available (commercial building).  Hard to believe the building would have a flat roof with absolutely nothing on it.


Posts: 90

« Reply #8 on: July 18, 2006, 07:51:45 AM »

From what the photos show, there is only a few short stakcs for venting and the gutter cages. The AC unit is in a room with the fan/compressor outside on the ground. The rest is plain.

Posts: 21764

« Reply #9 on: July 18, 2006, 08:08:48 AM »

You really need to get up on the roof and survey it.  I wouldn't "plan" anything at all without getting up there, making some sketches, taking notes on exactly what is there and how it might be used, etc.

As I posted earlier, if the roof is covered with insulating foam (very common on flat roofs, it's great stuff and lasts a very long time), you can walk on that just fine but it's kind of "spongey" feeling.  Problem is, if you wanted to use the metal roof under the sealant as a ground plane for a vertical antenna, you'd have to cut through the foam to access the metal roof, and that might be a big issue.

However, if the roof really is flat and all metal, then horizontal wires won't work well up there.  They'll behave as if they're that same height above earth ground; so an HF dipole should be at least thirty feet above the roof to be effective on 20 meters; it really should be higher than that for 40 meters.

Offhand, I'd probably figure out a way to use existing vent pipes to support masts.  If it's an industrial building, those are probably not PVC or composite, but really metal (not guaranteed, you'd really have to check) and might be very strong.  One might make a good base support for a real HF vertical antenna which can be guyed with lightweight Dacron ropes to further support it.


Posts: 39

« Reply #10 on: July 20, 2006, 04:01:13 PM »

How about a Force12 Sigma 5  or the new Falcon J5 for 20-10 meters. It sounds like just the thing for your situation.

Good luck

Posts: 1490

« Reply #11 on: July 27, 2006, 06:52:06 AM »

If you really can't get a good low-impedance connection to the roof (though I would bet you can), your best option might be to put up a wire antenna (sloping or horizontal doublet, ladder-line fed) attached to the edge of the roof or to a mast to get it even higher, and running away from the building.  That way you will benefit from the height of the roof and may see no de-tuning or other effects on your antenna.  The vertical with ground connection to the roof, located near the center if possible, is probably still the best way to go, however.

You did say it has to be a wire antenna -- what about a wire vertical inside some PVC pipe?  You can paint the PVC gray to protect it from the sun etc. and guy it in the middle of the roof to make it look just like another vent stack.  Pick a good length that isn't a half wavelength on any ham band, but that is as long as you can put up (maybe around 14'? - I haven't done the math) and your tuner will handle it OK.  

You can also make the ends of the guys into a top hat to make the vertical tune even more easily - just connect wires several feet long to the wire inside the PVC (at a joint for ease of connection), with insulators at their ends and then dacron line the rest of the way from the insulators to the guy anchors (I've gotten away with anchoring to other vent pipes or the roof edge so I didn't have to make any holes in the roof covering).  That would make a killer antenna that can cover many bands and have a low radiation angle for good DX performance.
Best rx & 73 de kt8k - Tim

Posts: 17480

« Reply #12 on: August 08, 2006, 01:57:31 PM »

I recently helped to put up a 80m dipole 10' above a large
flat metal roof, tied between an AC unit and a vent pipe.  
It actually worked better than the same antenna over ground
because the metal roof had lower losses.  The biggest
problem was that the feedpoint impedance was very low - I
think we used a 1uH shunt coil across the feedpoint to
match it.  So wire dipoles for 40 and 80m should be quite
practical - these are the bands where high angle radiation
is still useful for local contacts.

Above that I'd go for a vertical.  I'm presuming that the
requirement for wire antennas is related to visibility.
I'd look for a very thin whip antenna.  Perhaps the first
step is to walk around the outside of the building (and
the approaching road) to see just how visible antennas would
be up there: the roof I worked on had about a 4' wall
around it, so anything lower than about 6' high couldn't
be seen from the ground.  So consider something like a
CB whip, preferably with an extension on the bottom to
make it as tall as possible.  Paint it black to make it
less noticable.  The whips used on 2m 5/8 wave whips are
very thin and can't be seen from a distance - perhaps
you can come up with a combination with one of those
at the top?  If you can manage, say, 12 feet high, that
should work well enough on 20 - 10m.

If you can't connect to the metal roof, get a roll of
flashing metal and lay it down on the roof coating as
two or three sets of radials - this should give adequate
capacitive coupling to the metal underneath.  A bolt
through these will allow connection to them.  For a
mount you can make one or buy one designed to take a
cinderblock, sand bags, or regular bricks that can be
carried up to the roof a few at a time and used to weight
down the mount.  (I've also seen plastic dishpans filled
with concrete used to anchor roof tripods.)  This should
give you a stable vertical whip that is hard to see from
the ground, yet gives good performance with an autotuner
on the higher bands, all without doing any damage to
the coating on the roof.

Posts: 352

« Reply #13 on: August 16, 2006, 07:32:36 AM »

I would think that a radio shop would like anice big apparent antenna on top if just simply to impress the straights. Smiley

Get a wire as far away from that building as you can.  Standoffs along a metal building will simply not radiate much better than a dummy load.  (I tried that in the early years.)  Even a very slight wire out to a tree would be better.

You could load gutters and such, but the chances of them being ungrounded are low.

Also make sure you ground the rig at RF or at least some counterpoises.  100W may give a nasty RF burn without it.

The loop is not a good idea, IMHO.

73, JP, K8AG
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