Re: Need some antenna advice (for my CM15A, of course)

[nobody@xxxxxxxxxxxx (Dave Houston)]

"Michael G." <514mike@xxxxxxxxx> wrote:

>1 - If a cable is RG58U, does that mean it's automatically 50 Ohms?  I
>have some RG6 cable here, and one is labelled as 75 Ohms, so I know
>that's not optimal, as you mentioned.  If, however I find RG58, then am
>I okay?

RG58 is 50 ohm; RG6 & RG59 are 75 ohm. 50 ohm is optimal in this
application.

>2 - How bad is it to use F connectors instead of BNC connectors?  It
>seems like I have so many of these things lying around, and I have a
>crimper and all sorts of accessories in my workroom.  Can I put F
>connectors everywhere instead of the BNC ones?

F connectors are 75 ohm. 50 ohm BNC is optimal in this application.

>3 - For the phasing line, if RG62 (93-ohm) coax is not available, what
>type of coax would you recommend as my second and third choices?  Will
>I then have to cut it longer or shorter to compensate?

75 ohm coax would be the next best choice. Keep the length as stated on the
web page. The nominal impedance of a loop antenna is 100 ohms. The phasing
line is intended to match the 100 ohms as close as possible. The loops are
mounted perpendicular to each other. The signal from one will be 90° out of
phase with the signal from the other. The phasing line (length=wavelength/4)
acts as a delay line to bring the signals in phase. The two 100 ohm loops
are connected in parallel so the resulting impedance is approximately 50
ohms which matches the 50 ohm RG58 coax.

>4 - Is there any trick when it comes to dealing with the ground part
>(the shielding) of coax cable?  I just kind of strip it, and twist up
>all the little threads and tin them.  It looks silly, but is that the
>correct thing to do?  It's a little challenging soldering the thin coax
>cable they send with the kit onto the board.  I'd love to just solder
>some type of surface-mount F connector right onto the PCB.  In the
>picture on their website, it has that.  this just cable with one foot
>of cable.  Soldering it on is a problem.  Advice is welcome!

I use an awl to debraid the coax, twist the strands together and tin with a
soldering iron.

I just use a couple of short pieces of hookup wire to connect the preamp to
bulkhead type BNC connectors. It's not optimal but...

>5 - Must the TenTec Amp be placed as close as possible to the
>eggbeater, or can I mount it down by the CM15A?  Intuitively, I'd think
>it has to be closer to the antenna, but perhaps not.  I'd love to keep
>the amp near the CM15A.  Your thoughts?

In theory, the preamp should be at the antenna. In practice, it works fine
if it's near the CM15A.

>6 - Is there a limit of length for the coax between the CM15A and the
>eggbeater?  I would love to leave the CM15A here in my basement, yet
>run the coax up the side of my house, and place the antenna in my
>attic.  Is that okay?  (This is why point #5 is also important.  I have
>no problems running coax up to the attic, but I'll have no power for
>the power supply!

The length of the coax (within reason) is not a factor. Your proposed setup
is fine.

>7 - Last, but not least, from a physics point of view, is this the best
>type of antenna to construct?  I know other companies make antennas for
>this sort of thing, but they are straight.  Also, my old transceivers
>have those telescoping ones (I actually attached larger ones), and they
>work like a charm.  Is the eggbeater style truly the best possible
>shape?

There are about as many antenna designs as there are fish in the sea. A lot
of people have used a turnstile design made from twinlead that I documented
a few years ago. The advantage of the eggbeater is that it is easy to build,
is omni-directional (more or less) and has 50 ohm impedance (so no balun is
needed). My tests have shown it to be measurably better for this application
than the telescoping monopole, ground plain, turnstile, Lindenblad, and
other DIY designs. Eggbeaters are widely used to receive data from weather
satellites.

However, environmental factors may influence reception. IOW, YMMV.