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Old 3-Jun-2020, 1:37 PM   #11
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 153
Quote:
Originally Posted by bobsgarage View Post
So, I ask the members here. If my 1/4 wave matching section isn't doing the trick,
what should I do?
Most antenna analysis considers the antenna is in free space.
That is far away from earth or any electromagnetic material (conductors in particular).

Stacking antennas close to earth (or things) without precise measurements
of the environment (beyond hobby capability) is unlikely to provide the
desired theoretical gain values. This is particularly true for vertical stacking.

For good results both antennas must see the same signal wavefront.
This is not likely with vertical stacking at common heights,
and may not happen with horizontal stacking.

The likelihood of dissimilar signal wavefronts for each antenna,
plus losses in the combiner, makes stacking antennas for increased gain
an uncertain proposition.

However, horizontal stacking often has the best chance for satisfactory results.

Quote:
… Do the 1/4 wave transformers have a very narrow band?
In theory, 1/4 wave transformers are narrowband.
The term ‘narrowband’ probably means different things to different people.
But, most would consider the UHF TV band to be a wide band
(approaching 30%).

Quarter wave transformers match impedance perfectly at only one frequency.
At other frequencies the impedance match degrades with distance from the center.

Some may say that the loss (dB) of a quarter wave transformer is not very
significant over the UHF TV band. That usually is true.
However the impedance mismatch can be significant.
Impedance mismatch causes signal rejection (mismatch loss).

Most TV antennas and preamp/tuners have relative poor impedance control
(not 75 Ohms).
The performance of a mismatch quarter wave transformer working against
a mismatched preamp/antenna combination (without deeper considerations
and measurements) is often unpredictable

Nevertheless, a quarter wave transformer could provide better performance
compared to a ferrite-based splitter.
This is mainly because ferrite-based splitters have some inherent loss
(independent of mismatch) and some splitters also have appreciable
impedance mismatch.

Some of the difficulty of Quarter Wave transformers at UHF is the impedance variation
at the mechanical/electrical joint

Quote:
…Are they even practical?
For most, achieving theoretical results probably is not practical,
but, ‘one may get close enough for practical purposes’.

Quote:
…I sure don't want to go to the trouble of combining
two VHF or two UHF antennas only to lose some gain due to combiner loss.
Some combiner loss is unavoidable.
It is the amount of loss that is of most concern.

Quote:
…What's the deal with the Ferrite beads?
Ferrite is often used to attenuate signal strength,
there are all kinds of beads available for that purpose.

Commercial baluns and many signal combiner/splittters use ferrite as a core
to help provide transformer action (inductance etc).
Manufacturers select particular kinds of ferrite that are less lossy and also
have capability to increase inductance etc.
The use of ferrite is a tradeoff.
Without the ferrite, the commercial baluns and splitters could be either larger
(if wound with air or other core) or more narrowband, or both.

.
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