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Old 25-Jun-2013, 1:52 PM   #29
Senior Member
Join Date: Jan 2010
Location: Dallas, TX
Posts: 126

Thanks Pete.

Originally Posted by Pete Higgins View Post
... Did you ever implement the coax balun?...
Yes, a CM-0264 was physically modified to remove the internal balun and replace it with an internal coaxial half-wave loop balun.

Two types of lab tests were performed:

1. Gain vs. frequency response (dB)- Results before and after the modifications were compared.
2. Post-detection S/N (dB)- Using off-the-air DTV signals, both the modified and an unmodified CM-0264 were measured and compared.

No, the modified preamp has not been installed on the main tower.

From what I’ve read, coax balun’s have a very narrow bandwidth so I would have thought even 24 MHz (ch. 20 -23) might be too wide?
Depending on how one defines bandwidth, a half wave loop could be considered to have narrow bandwidth relative to ferrite core baluns. However, for use in TV reception the half wave loop balun has a fairly wide usable frequency range. The losses of a half wave balun depend, as does a ferrite core balun, on design and construction. If one considers usable bandwidth to be when signal loss of a half wave balun is comparable to that of a conventional balun, then it probably has a usable bandwidth approximately equal to the current UHF band (Channels 14 –51).

It may be instinctive to think of functions that require specific wavelengths of coax to be narrow in bandwidth. This thinking could arise because such coaxial items can be a specific length (in terms of wavelength) at only one frequency. Only one frequency, that does seem narrow

The above is true if the objective depends on signal cancellation (e.g. notch or trap filters). For signals to nearly cancel, the amplitudes must be near exact and the phase relationship (wavelength-related) must be nearly exact.

But, the function of a half wave loop balun depends not on signal cancellation (subtraction, or out-of-phase condition) but instead utilizes in-phase signal addition. The half-wave delay (180 degrees) of a loop balun attempts to align (in phase) the normally out-of-phase signals of the left and right halves of a dipole antenna.

Even if their phase and amplitudes are not perfectly aligned, signals (vectors) can add with relatively small loss. When adding misaligned signals there is loss, but over a usable range the losses increase somewhat gradually as the misalignments worsen.

The main losses (in no particular order) associated with half-wave loop baluns:

1. Phase alignment Loss (Imperfect 180 degree delay)
2. Amplitude Loss (Normal coaxial attenuation for the half-wave section)
3. Impedance mismatch (Resulting from impedance transformation of non half-wave section)

Bottom line: Over much the UHF band (470-698 MHz), a well designed half wave loop balun can have lower signal losses than a ferrite-core balun and can at worst (near band edges) be comparable to good commercial ferrite core baluns.

Or, did you wind one on a toroid core?
Not in this particular case. It is essentially open-air loop, in close proximity to PCB plane and plastic housing.

Having wound many ferrite core baluns for VHF and UHF, winding twisted-pair on a 2-hole ferrite core could be an alternative, especially if coverage of channels at both both band-edges (near channels 14 and 51) is required.

I scavenged a VHF/UHF combo antenna …who knows what that might turn into one day.
Yes. you may find a good use for it.

I assume you weren’t in Dallas when you did this?
All the antenna work, discussed so far, was done in KY.

The preamp modification (balun related) and testing was done in Dallas.

The modified preamp is now stored in KY.

Last edited by tripelo; 25-Jun-2013 at 2:38 PM. Reason: typo
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