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Old 18-Apr-2020, 1:52 PM   #16
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 173
Quote:
Originally Posted by bobsgarage View Post
... I'm designing my own lossless combiner.
Interesting idea, using RG8 with RG11 connector.

Quote:
... filling the void inside the combiner. Epoxy? RTV? Liquid Tape?
Guessing from your image, probably RTV silicone would be a good candidate.
(Avoid silicone that contains acid, some use acetic acid, smells like vinegar)

Two things to consider:

1. Waterproofing
2. Impedance & Loss

Waterproofing, two approaches:

* Externally sealing the closed case. Any of the three items would probably work.

* Internally sealing. Also any of the three items, but there can be impedance
and loss considerations.

2. Impedance & Loss

If open internally, sealed externally, air dielectric, low loss, but may result in
high impedance due to wide spacing and small wire size.

Silicone has relatively low loss and dielectric higher than air. when using
relatively small wires, can help lower impedance.

For your combiner, the length of the wires internally are relatively short at VHF
frequencies, so impedance bumps (near the connections) may not cause a
noticable effect on performance.

------------------------------------

From another thread:

http://forum.tvfool.com/showpost.php...13&postcount=3

Quote:
... I am on a pause for now. Any new Ideas ?
No new ideas.

For a Yagi, if other dimensions are suitable,
the length of a populated boom has most effect on gain.

For wideband Yagis, such as for upper VHF, director spacing (within limits)
has small effect.

Director length is sized to accommodate the highest desired channel.
If director is too long, gain on upper channels will be lost.

----------------------

The 30-2476 was designed for channels above the US VHF band, so the
directors could be slightly lengthened to obtain more gain in lower VHF
channels.

One can model such things with NEC2, but to verify the performance, can
require a very stable signal source (such as antenna test range).

The change in gain would probably be difficult to verify with reception
of time varying signals (broadcast stations).

If one is within LOS of broadcast stations and know the expected signal variations.
One could make many measurements of the modified antenna and compare the
average to the average of many unmodified measurements.

If the modification involved lengthening directors, and wished to preserve
coverage of upper VHF, it would be good to at least test gain on
channel 13.

.

Last edited by tripelo; 18-Apr-2020 at 2:27 PM. Reason: Clarify RTV silicone, director length test
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