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Author Topic: TRSB with Buddistick  (Read 4713 times)

Posts: 31

« on: September 16, 2012, 08:33:38 PM »

Hello All,

Does anyone have experience using the TRSB (Triple Ratio Switch Balun) with their Buddistick?  If so, how did you attach it to the antenna?

I clearly need a balun for 20m with this antenna.

Glenn KG6SII

Posts: 5483


« Reply #1 on: September 16, 2012, 08:47:45 PM »

I clearly need a balun for 20m with this antenna.

A buddistick is an unbalanced antenna.  Why the need for a balun?

Mark K5LXP
Albuquerque, NM

Posts: 31

« Reply #2 on: September 24, 2012, 08:31:49 PM »

A buddistick is an unbalanced antenna.  Why the need for a balun?

K5LXP, I believe the feed-point impedance of the Buddistick is too low to match my 50ohm coax, so I'm losing power between the coax and antenna.  I was mountain-topping on my property the other evening and I could hear tons of other stations (PSK31 on 20m) but nobody could hear me, despite my transceiver's meter showing about 10watts output with a very respectable SWR.  Here's what the Buddipole fieldbook says:

20m Buddistick
The Buddistick should present about 6 ohms of radiation resistance for
this band. A full-sized radiator for the 20m band would be over 16 feet
in length. The radiator on the Buddistick is only about 7 feet—very short
for a vertical for this band. Shorter antennas are more difficult to match,
have smaller 2:1 SWR bandwidths, are less efficient than larger
alternatives.  The extremely low radiation resistance means that the feedpoint impedance at
resonance will also be very low. Even with ground losses the low
feedpoint impedance still gives an SWR greater than 2:1. (A Triple Ratio
Switch Balun setting of 2:1 or 4:1 matches this nicely, however.)

Posts: 7718

« Reply #3 on: September 24, 2012, 09:02:09 PM »

You measured a low VSWR therefore the antenna needs no matching.

Why does your Buddipole present a high (~50 ohm) input impedance even though the radiation resistance is low? Because of loading coil loss and ground loss. What you see is normal.

Now if you want to increase your signal use ten or more radials 8' in length. Now the ground losses may be low enough to where impedance matching is needed. If so, matching can be performed using a shunt inductor, the TRSB balun, or the variable matching transformer from MFJ.
« Last Edit: September 24, 2012, 09:14:48 PM by WX7G » Logged

Posts: 17484

« Reply #4 on: September 24, 2012, 11:21:20 PM »

So if you saw an SWR of 50 / 6 = 8 : 1, the antenna is presenting a 6 ohm load.

If the SWR is, say, 1.5 : 1 then the feedpoint impedance is between 33 and 75 ohms.
Which very well may be the case.

The radiator has a 6 ohms of radiation resistance.  What is causing the other 27 to 69
ohms of resistance in series with it is loss:  some in the loading coil, but likely a lot in
the ground system.  Let's say that the total impedance is 36 ohms:  6 ohms of that
is radiation resistance, which results in useful radiation.  30 ohms is loss, which doesn't
Your overall efficiency is 6/36 = 17%.

If the SWR is already usable, further matching won't help.  What you need to do is
to reduce the losses.  Improving the ground system is a good starting point.

If you cut the ground loss by 2/3 down to 10 ohms, the input impedance will now
be 6 + 10 = 16 ohms, for an SWR of about 3 : 1.  Your efficiency is now 6/16 = 37%,
which is a significant improvement.  But now you need to match the SWR, and an
un-un should work for you.

But if the SWR is good now, adding the matching transformer isn't going to help, it
will just make the SWR worse.  You have to improve efficiency, which will make
the SWR worse, before needing to provide any matching.

Sometimes it seems counterintuitive, but that's the way math works:  a good
SWR on a short antenna often masks high losses.  Correcting the losses
makes the SWR worse, but improves your signal.
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