[TowerTalk] Antenna Gain and Reality

Sat Dec 13 00:49:21 EST 2014

On 12/12/2014 7:53 AM, Lizeth Norman wrote:

Hi Lizeth,

This is my take "in plain language"and undoubtedly there are others
All passive antennas gain, gain, or it'd be better to say they achieve 
gain by removing signal from all but the desired direction in various 
amounts. Some of those amounts vary widely over the sphere.

The big, or huge difference between HF domain and the higher VHF and up 
domains is the amount of interaction with the terrain.  The Radiation 
angles with HF in all directions including f/b are dependent on the 
terrain and its makeup out to quite a distance. UHF and up, see terrain 
more as a reflective object, but their height above ground is many 
wavelengths so I'd expect them to behave more like an antenna in free 
space with a mirror far below...BUT they see the atmosphere as an 
absorber, with buildings being more like they are constructed of mirrors.

I'd think Cosmic radiation would make for a relatively steady signal 
source (depending on where you are pointed)  I don't know much about the 
construction of UHF and up, antennas.

I believe UHF and up see the horizon and hills as refractive objects.

When I lived South of  Breckenridge, MI, the Edmore repeater on 2-meters 
was well over the horizon and beyond where I'd expect reliable 
communications.  Drawing out the elevation profile including the earth's 
curvature showed a rather sharp ridge between us, a little closer to 
them than me.  Communications were always good with a mediocre antenna 
at 20 to 30 feet.  Not quite good enough for consistent mobile work.  I 
was surprised at the width of acceptable reception and they received VHF 
stations well out to 50 miles.

When TV went digital, I had a pair of long UHF Yagi antennas with corner 
reflectors  (Radio Shack models. Although a bit fragile they worked as 
good as the high priced spread.)
  http://www.rogerhalstead.com/ham_files/Tower26.htm.  The one pointed 
NW is prominent. The one to the SSE is invisible in the photo.  It's 
highly visible in http://www.rogerhalstead.com/ham_files/tower41.htm, 
but the one to the NW is above the frame.  At 90 to 95 feet these 
antennas reached the Detroit to Lansing/Jackson area to the S and 
Traverse city to the NW with solid coverage.  I did have antenna mounted 
preamps due to the very long coax runs, but I had 20 to 22 UHF digital 
stations that were solid into my location.  Not much worth watching, but 
I had 22.  The top mounted 144 and 440 Yagi antennas gave comparable 
performance.  Unfortunately at 30 feet of mast between them ant the 
thrust bearing, the flexing of the mast beat the snot out of them. I was 
surprised that all it really did was tip the elevation angles on the 
antennas. The left, or port 2-meter Yagi ended up pointing about 30 
degrees above the horizon.  OTOH it did destroy the TB-3 thrust bearing.

73

Roger (K8RI)

> Hi Roger!
> Good to hear you again. All you say is true with HF antennas. On the
> other hand, what about VHF and higher?
>
> Looking around I've found a bunch of stuff on optimizing and
> qualifying antennas by using solar/cosmic noise. Every explanation
> (makes a bunch of sense) that I've read says that the characteristics
> (f/b, g/t)of the antenna are necessary to proceed with the
> measurements.
>
> As it's been said bunches of times, the figures provided by
> manufacturers are suspect at best.
>
> K3NG has an arduino rotator control project that has a sun tracking
> feature, which almost automates the qualification process, if I
> understand the process correctly.
>
> What to do?
>
> Constructing an antenna range is a possibility. There is a "how-to" in
> a piece of software called HDL (horn, dish, lens).
>
> Is this worth it? Time is fun to waste but I'd like something to show for it.
>
> Norm n3ykf
>
> On Thu, Dec 11, 2014 at 7:51 PM, Roger (K8RI) on TT
> <K8RI-on-TowerTalk at tm.net> wrote:
>> As I see it:
>> There are several absolutes in antenna gain and many probably, might, maybe,
>> could be attributes.
>>
>> The two absolutes are the 2.1 db dipole to Isotropic source (I've been a ham
>> since 61 and the books have always listed that figure) and gain measurements
>> are only true for the test range where they were made.  Hence even though
>> the manufacturer may be honest and make an honest attempt at measuring gain
>> figures in all axises, comparing one manufacturers antennas against another
>> manufacturers antennas from advertized data only gives ball park figures and
>> tells me little about how they will perform in my installation.
>>
>> Doubling the number of antennas theoretically doubles the gain which is an
>> additional 3 db for each doubling although it's unlikely due to losses in
>> feeding and matching that this will be achieved. Losses in the additional
>> coax can be substantial.
>>
>> Even with computer modeling of the installation, the inputs must be accurate
>> and few if any know the ground characteristics near or far to any great
>> precision.
>>
>> The radiation pattern at vertical angles is a crap shoot.  You are playing
>> the percentages for atmospheric conditions that will match the maximum
>> signal radiated will be at the most desirable angle to match those
>> conditions.  That's why the contesters have stacks that allow them to select
>> the antenna height that lets them put the maximum signal into the desired
>> range/distance.
>>
>> As most of us have both budget and land constraints that limit us to the
>> number, size, and height of our antenna(s) It's unlikely we can depend on
>> equaling the performance in the advertized figures.  Often for 40, 75, and
>> 160, a simple wire antenna, or vertical will out perform all but the largest
>> and/or sophisticated antenna or array.
>>
>> I've had extremely good luck with sloping, center fed, half wave dipoles.
>> compared to some stations running Yagi antennas at reasonable heights. Many
>> times the first word in the response to my call, is "Wow".  Yes, I do run
>> QRO, but the best sounding signal out of my tetrode amp is at, or around the
>> legal limit so there is little incentive for pushing beyond the legal limit
>> for that extra 1 db and those tubes are expensive.
>>
>> --
>>
>> 73
>>
>> Roger (K8RI)
>>
>>
>> ---
>> This email has been checked for viruses by Avast antivirus software.
>> http://www.avast.com
>>
>>
>> _______________________________________________
>>
>>
>>
>> _______________________________________________
>> TowerTalk mailing list
>> TowerTalk at contesting.com
>> http://lists.contesting.com/mailman/listinfo/towertalk
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk at contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk

-- 

73

Roger (K8RI)

---
This email has been checked for viruses by Avast antivirus software.
http://www.avast.com