[HCARC] Antenna Questions Of The Day

[Kerry Sandstrom]

Hi Gary,

The diameter does make a difference.  Larger diameter tubing will have a 
wider SWR bandwidth and the antenna will be a little shorter.  There are 
formulas for calculating the length for different diameters and they may 
even be in the antenna book or ARRL Handbook.  For reasonable tubing, its 
just a couple percent.  If you use the standard length formula and calculate 
the length for the high end of the band, that will probably be good enough 
except on 80 meters.  The dimensions should be in wavelengths which should 
dramatize the fact that for HF any sensible tubing is very small.

What its made of doesn't really matter within reason.  Poor conductors will 
have some additional loss but I don't you can notice it.  The wire will be 
narrower SWR bandwidth but if you use an antenna tuner it wont matter. 
Copperweld, copper, and aluminum wire are all used successfully.

Yes tapered sections will have a length that is in between the length of the 
biggest tubing and the length of the smallest tubing, but again the 
difference is great.

The point of the roller inductor is to add inductance so the effect is 
actually a lot bigger than just the length of the wire.  When you are 
matching a load like an antenna to a transmitter, there are two goals: 
first, you want to cancel out the reactive component and, second, you want 
to transform the load impedance to the output impedance of the transmitter. 
It doesn't matter which order you do it in.  Depending on its length, an 
antenna impedance can look inductive, capacitive or resistive.  Usually its 
got some reactance and some resistance.  If the antenna is capacitive, you 
need to use inductance to cancel it out so you have a pure resistance.  An 
antenna tuner does both the cancel the reactance and transform the 
impedance.  It typically has 3 variable components, two capacitors and an 
inductor.  There are several possible configurations, however, they all will 
work.

Adding or removing 2 foot sections might be good at 80 m but is probably to 
big a step for the higher bands.  Personally I would make the antenna as 
long as I could and then rely on an antenna tuner to adjust it to the 
transmitter.

I'm sure you've heard me talk about "magic" before.  The 43 foot vertical 
falls into that category!  Arguably the best ham radio HF vertical is and 
has been for decades the Hy-Gain 18-HT  Hy-tower.  It is over 50 feet tall 
and has a couple sections of triangulatr tower at the base.  It covers 
80 -10 primarily the old bands although I think you can use it on 30 and 17 
m also.  The only reason I don't use one is they cost ~$1000.  A couple 
years ago people started pitching 43 feet as the optimum length for a 
vertical.  You have to use some kind of a tuner with this length.  A 66 foot 
vertical would be a better choice but is difficult to support!  In any 
event, the bigger the better, size does matter.  If its not an odd multiple 
of quarter wavelengths long, its going to need a matching device such as an 
antenna tuner.  Even if it is an odd multiple of quarter waves it is still 
going to need some kind of matching since its impedance is about 35 Ohms. 
A 33 foot vertical is resonant for 40 and 15 m but is too short for 80 
unless you use somekind of loading and/or matching.  You also need a 
matching system or loading to make it work on the other bands.  A 43 foot 
vertical would work better than a 33 foot vertical because it is bigger but 
it may not be as easy to match.  You probably would have a difficult time 
seeing the difference.

As I've said before, you can make anything work and with a tuner, it 
probably won't make much difference what you use.

Yes, I just saw your latest e-mail.  Remember the league is out to make 
money.  They have two ways, accepting donations or selling books and 
magazines.  I believe they produce books and magazines to make money and not 
to make a better life for hams.  Excellent antenna modeling software is 
available free from our government!  It is not and never will be simple to 
use however it is well documented.  A good model is simply a starting point. 
It will get you into the ballpark but even if you follow the design 
dimensions exactly it will still take some playing around to make it work. 
I haven't seen the book but I can't believe it will really make it any 
easier.  In order to do an accurate model of a real antenna you have to know 
what the ground parameters are, what conductive or lossy objects are near 
your antenna, The dimensions and configuration of everything conductive on 
your antenna, the characteristic of your ground system and transmission 
line, etc.  Beyond a point, modeling is counterproductive.  It will let you 
look at them impact of changes on performance, but the actual performance 
and the match in particular are something you best do experimentally.

Have fun,

Kerry