Jeff wrote:
I would think a dipole would be a bad candidate for a "short" antenna
as you need to get the matching stuff close to the antenna to avoid
I^2R losses.

Some sort of ground mounted, or close to the ground, antenna might
make a better choice.


A dipole may be a better candidate, if you use a monopole then although
you will be able to put your matching close to the antenna, providing an
effective earth screen becomes vital and earth losses may outweigh any
advantage.

Of course if you do use a dipole it is vital that any matching is at the
feed point and not on the end of a length of coax or the losses will soar.

The issue with dipoles is height.

As the height of a dipole decreases below 1/2 wavelength the pattern
elevation angle starts increasing and very quickly has most of the
energy going straight up.

At 160M that means getting the dipole 260 feet up in the air, and if
you can do that, why would you be concerned about a short antenna
as a full 1/4 wave 160M antenna is only about 140 feet tall?



--
Jim Pennino

On Thursday, November 6, 2014 12:46:04 PM UTC-6, wrote:
Jeff wrote:
I would think a dipole would be a bad candidate for a "short" antenna
as you need to get the matching stuff close to the antenna to avoid
I^2R losses.

Some sort of ground mounted, or close to the ground, antenna might
make a better choice.


A dipole may be a better candidate, if you use a monopole then although
you will be able to put your matching close to the antenna, providing an
effective earth screen becomes vital and earth losses may outweigh any
advantage.

Of course if you do use a dipole it is vital that any matching is at the
feed point and not on the end of a length of coax or the losses will soar.

The issue with dipoles is height.

As the height of a dipole decreases below 1/2 wavelength the pattern
elevation angle starts increasing and very quickly has most of the
energy going straight up.

At 160M that means getting the dipole 260 feet up in the air, and if
you can do that, why would you be concerned about a short antenna
as a full 1/4 wave 160M antenna is only about 140 feet tall?



--
Jim Pennino

When I was on 160m a lot, I had both the T vertical, and I also
had a "Z" dipole, which was the only way I could fit a dipole on
this lot without loading coils. In some cases for NVIS, the Z dipole
was a bit better, but overall, I prefer being vertical on 160.
On that band, the path doesn't really have to be that far for the
vertical to do as well or better than the very low dipole. And at
farther distances, the vertical will generally outperform the low
dipole by quite a large margin.

So if I could could only have one antenna, I would generally prefer
the vertical, because a dipole on that band really needs to be pretty
high to play well. My Z dipole was only at about 40 feet at the apex,
which is only slightly above 1/16 of a wave up. It was fairly
lame overall.

W8JI plays on 160m a lot, and has lots of good info about that band
and his various antennas on his web site, if it's still around.
I haven't looked at it in a while.

wrote:
On Thursday, November 6, 2014 12:46:04 PM UTC-6, wrote:
Jeff wrote:
I would think a dipole would be a bad candidate for a "short" antenna
as you need to get the matching stuff close to the antenna to avoid
I^2R losses.

Some sort of ground mounted, or close to the ground, antenna might
make a better choice.


A dipole may be a better candidate, if you use a monopole then although
you will be able to put your matching close to the antenna, providing an
effective earth screen becomes vital and earth losses may outweigh any
advantage.

Of course if you do use a dipole it is vital that any matching is at the
feed point and not on the end of a length of coax or the losses will soar.

The issue with dipoles is height.

As the height of a dipole decreases below 1/2 wavelength the pattern
elevation angle starts increasing and very quickly has most of the
energy going straight up.

At 160M that means getting the dipole 260 feet up in the air, and if
you can do that, why would you be concerned about a short antenna
as a full 1/4 wave 160M antenna is only about 140 feet tall?



--
Jim Pennino

When I was on 160m a lot, I had both the T vertical, and I also
had a "Z" dipole, which was the only way I could fit a dipole on
this lot without loading coils. In some cases for NVIS, the Z dipole
was a bit better, but overall, I prefer being vertical on 160.
On that band, the path doesn't really have to be that far for the
vertical to do as well or better than the very low dipole. And at
farther distances, the vertical will generally outperform the low
dipole by quite a large margin.

That is exactly what one would expect from the patterns.

So if I could could only have one antenna, I would generally prefer
the vertical, because a dipole on that band really needs to be pretty
high to play well. My Z dipole was only at about 40 feet at the apex,
which is only slightly above 1/16 of a wave up. It was fairly
lame overall.

W8JI plays on 160m a lot, and has lots of good info about that band
and his various antennas on his web site, if it's still around.
I haven't looked at it in a while.

I remember him. AIR he uses 1/4 verticals at home and was very
involved in mobile antenna contests at one point.



--
Jim Pennino

On 11/6/2014 10:56 AM, John S wrote:
On 11/6/2014 7:16 AM, amdx wrote:
On 11/5/2014 1:08 PM, wrote:
On Wednesday, November 5, 2014 1:01:02 PM UTC-6,

I would think a dipole would be a bad candidate for a "short" antenna
as you need to get the matching stuff close to the antenna to avoid
I^2R losses.

I'd probably use small matching coils at the feed point, about the
same as a mobile antenna.
In the real world, I try to avoid short antennas..
It's only a last resort due to lack of room. I rarely actually use one.


OK, nuff jabbering.
I'll set the rules 15ft tall, designed for 80 meters.
16 radials, base mounted inductor and whatever tophat you desire.
Do the theoretical design find the feed impedance with some efficiency
numbers.
Then compare data to a dipole at 1/4 wave height.

Ready Set GO!

Wait, do you have a better idea than a base mounted inductor?

As Mark said, maybe 50-75% along the antenna.

How long are the radials?

How much space can you give us?

Do you want to compare 8 to 16 radials.

We can do that. (Or you can do it yourself with EZNEC)

The tophat needs to survive 80 mile an hr wind.

I don't know how to do that.

I'm sure there's more.

Yes, always.

I don't have a radio so it is moot point to me. I was just trying to
get a design going, as there just seemed to be a lot of jabbering.
But, there are just so many variables and you do need to start with a
location, how much money are you going to spend, what materials
can you get, and then design around them.
I'm not an actual ham but I did read Jerry Sevicks book,
"The Short Vertical Antenna and Ground Radial"
http://tinyurl.com/p3rp2qe

Mikek

On Friday, November 7, 2014 11:22:30 PM UTC-6, amdx wrote:

I don't have a radio so it is moot point to me. I was just trying to
get a design going, as there just seemed to be a lot of jabbering.
But, there are just so many variables and you do need to start with a
location, how much money are you going to spend, what materials
can you get, and then design around them.
I'm not an actual ham but I did read Jerry Sevicks book,
"The Short Vertical Antenna and Ground Radial"
http://tinyurl.com/p3rp2qe

Mikek

With me, I just don't have time to do something I've already done
several times years ago.. I know pretty much from what I've already
modeled, what works and what doesn't. I used to have loads and loads
of stored antennas I modeled, but like I say, it's all on old drives
I don't use any more.
As I seem to recall, Jerry Sevick knows his stuff, so you won't go wrong
with that book.

amdx wrote in :

what materials
can you get, and then design around them.

I'll risk looking like a devil's advocate here and suggest good austenitic
stainless steel, like 316, for wires and radials, as a first base.
Reasons:
1. Cheaper than copper.
2. Stronger than copper.
3. If after a bit of stretching, a strong NdFeB magnet shows total
indifference to it when introduced, it permeability is likely low enough to
ignore if you're doing a first, empirical test of an antenna.
4. Very good chemical resistance, eliminating a whole heap of environmental
concerns for its endurance and performance.
5. Very good physical resistance, so no need to cover with insulator, thus no
need to add that into high frequency modelling.
6. Easy to find all over eBay...

I'm sure there are things I didn't think of, and if there is some vitally
specific reason not to do this, you'll likely already know it, but my point
is that if you just want to get something tried out first, economically, it
is likely better this way than starting with copper, for the simple reason
that if it fails, the stainless stuff will do you good service in some other
way, sometime, and wasted copper is less useful, and loses money very fast.

After posting, I remembered one important one, electrical resistance. That
might strongly reject austenitic stainless as a choice, if the antenna is
small or otherwise makes low resistance critical.

On the other hand its high strwength to weight ratio might be another strong
plus to add to any others, it can save nasty accidents with underestimating
wind loads, breaking strains and such when testing a new build. (For wires,
I'm less sure about towers, I haven't learned enough to make suggestions for
materials and structural forms and dimensions for those).

On 11/8/2014 5:22 AM, Lostgallifreyan wrote:
After posting, I remembered one important one, electrical resistance. That
might strongly reject austenitic stainless as a choice, if the antenna is
small or otherwise makes low resistance critical.

As far as I can tell by modelling, SS does have a bit higher resistance
than copper, but it is the permeability that kills it for a radiator. I
would definitely go for low-permeability SS over copper in an
appropriate situation.

On the other hand its high strwength to weight ratio might be another strong
plus to add to any others, it can save nasty accidents with underestimating
wind loads, breaking strains and such when testing a new build. (For wires,
I'm less sure about towers, I haven't learned enough to make suggestions for
materials and structural forms and dimensions for those).

Lostgallifreyan wrote:
After posting, I remembered one important one, electrical resistance. That
might strongly reject austenitic stainless as a choice, if the antenna is
small or otherwise makes low resistance critical.

On the other hand its high strwength to weight ratio might be another strong
plus to add to any others, it can save nasty accidents with underestimating
wind loads, breaking strains and such when testing a new build. (For wires,
I'm less sure about towers, I haven't learned enough to make suggestions for
materials and structural forms and dimensions for those).

The resistivity of stainless steel is about 35 times that of copper and
about 20 times that of aluminum.

This will make a big difference for a wire antenna, but less for one
of fat tubing or rod, however stainless tubing is both heavy and expensive.

It would all be a bunch of engineering trade offs that would depend on
what one wants to accomplish.

If it happens that your brother-in-law can get you stainless at a good
price...


--
Jim Pennino

wrote in :

The resistivity of stainless steel is about 35 times that of copper and
about 20 times that of aluminum.

This will make a big difference for a wire antenna, but less for one
of fat tubing or rod, however stainless tubing is both heavy and expensive.

It would all be a bunch of engineering trade offs that would depend on
what one wants to accomplish.

If it happens that your brother-in-law can get you stainless at a good
price...


I have no brother-in-law. But I usually have a bit of stainless steel
around.

Ok, so no high current transmitters unless some sturdy mechanical design is
used. What about QRP? I understand that high efficiency and a large antenna
is best for very low power transmitters, and given the lower currents
involved, would I^2R losses be low enough to be pretty much as viable as for
reception? This strays from the short antenna subject, but I'm interested
enough to ask anyway.

(If I needed to use tubing, I'd consider aluminium for lighter weight
combined with more stiffness.)