Re: Topband: 160 Meter BALUNS

[Joe Reisert <W1JR@arrl.net>]

Hi Tom,

A follow up to my prior EMail on a solenoid baluns.

Regardless of whether 50 feet of RG8X wound on a solenoid (tube) is proper 
to use a balun or not, I'll leave that up to you and W8JI to decide. I 
guess you some would describe this type of balun as a choke.

Now, here is some more theoretical information and measurements etc. that 
maybe of interest to some for the engineering types on this reflector. This 
maybe helpful to design similar types of solenoid baluns at other frequency 
bands.

50 feet of RG8X coax is a good starting point for a 160 meter solenoid 
(choke) balun for many reasons. If you close wind the coax on a 4.5" OD (I 
mistakenly said 4" in my prior EMail) standard white PVC tube, you obtain 
an impedance of about 650 Ohms (as measured on an HP Network analyzer). 
This means that you are above the 500 Ohms (and well above 250 Ohms) 
impedance that most experts feel is adequate for a balun impedance.

At 80 meters, the same solenoid balun will have an measured impedance of 
about 1300 Ohms. However, depending on how tight you make the turns, a 
resonance will be noted somewhere between 12 and 15 MHz. Hence, 50 feet of 
coax is probably only good for 160 through 30 meters. Use less turns if 
only for higher bands (see below). It looks like a 4 or 5:1 ratio of lowest 
frequency to highest useable frequency is a good rule of thumb.

To carry on further, 50 feet of RG8X will amount to approximately 38 turns 
on a 4.5" OD former and the winding will be approximately 9.25" long. If 
you plug these numbers is into most standard equations to calculate 
inductance, you will calculate an inductance of approximately 60 micro 
Henries. Using the standard formula for reactance: Xl = 2 pi FL, yields 
about the same impedance as measured above. Pretty nifty to get such 
agreement! So, you can see that don't need fancy measuring gear to make a 
solenoid balun for any band. Just decide on how high an impedance you want 
(but not too high-see below) and make sure that you don't put on too many 
turns so resonances will occur above rather than in band!

Some may ask if RG58 is OK for a solenoid balun. Sure it is but for lower 
power than RG8X. Since it is slightly smaller in diameter, 50 feet have a 
slightly higher impedance. Since the power handling ability of coax goes 
down as frequency increases, it maybe safer to use Teflon (RTM) type coax 
such as RG303 if you are running high power, especially at 80 meters and 
above. RG8 will also be OK but it is larger in diameter so more coax will 
be required. You can make your own calculations on this one. I wouldn't 
recommend foam RG8 coax as it may deform on such a small diameter. However, 
if you use a larger diameter tube, that will work with RG8 and since the 
diameter is larger, the impedance will increase accordingly. Use the 
standard inductance equations.

I made a solenoid balun with 25 feet of RG303 teflon (RTM), about 20 turns 
on a 4.5" tube, and the first measured resonance was about 24 MHz. This 
balun would be great, even at high power, for 80 through 15 meters. Again, 
about a 5:1 frequency range.

Some purists will say to space the turns, for example, by the diameter of 
the coax. This maybe less of a problem for flash over if lightning hits. 
I'll leave that up to you to decide. However, using the info above, this 
would calculate (using standard inductor equations) to about 33 micro 
Henries of inductance for space windings (and an impedance of only 375 
Ohms), well below that normally suggested for a 160 meter balun. Hence, 
more coax or a larger diameter tube is required.

Finally, what about laying the balun on the ground. I'd recommend against 
that simply because that at least may lower the self resonance frequency. 
This is the old story that you shouldn't place objects near (1-3 diameters 
away) an inductor (which is what a solenoid balun is on the outside shield).

I hope this info is of interest and help. There will always be the 
disagreements over whether to use ferrite beads, ferrite toroids or 
solenoid baluns. No one size fits all! However, for those interested in 
designing their your own solenoid type baluns, I've hopefully given some 
info on how to "roll your own."

Happy holidays and best of DX in 2004.

73,

Joe, W1JR

At 10:24 PM 12/19/2003, TNeill1648@aol.com wrote:
> The thread on ferrite beads has been interesting.  Tom, W8JI, suggest the air
> core wound BALUN for 160.  Can anyone give me the diameter and number of
> turns of RG-213 or equivalent to use for a 160 air wound  BALUN?  Thank you.
>
> Tom Neill, K4XG
> _______________________________________________
> Topband mailing list
> Topband@contesting.com
> http://lists.contesting.com/mailman/listinfo/topband

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