[TowerTalk] 160m project

[K4SAV]

Lee

Here are a few items to consider about your planned 160 antenna. 

Warning: This post is long and has a lot of detail which may bore some 
of the readers, but I'm sure Lee will be interested.

Lee said: ..."A mil-spec balun from Antenna Products rated at 2-30M 4Kw PEP
http://www.antennaproducts.com/BINDEX.htm<about:blank> SM-153."...........

Where did you find the data on this balun?  All I could find was on 
their website and it said 1 KW av / 2KW PEP.  There was no other info, 
no mention of what type balun it is.  You said it was MIL Spec.  I don't 
see that on the website or any MIL Spec numbers.  Google didn't turn up 
anything either.  The website says it has an LC input connector.  What 
is that?  I wouldn't buy a pig in a poke (unless I knew it was really a 
pig).  Have you checked out the DX Engineering baluns?

Lee said: ....9931 coax.........

I am not familiar with 9931 coax.  Did you mean to say 9913?  If so, it 
would not be my choice for 160 meters.  It's low loss but if you let 
water get into it, it's trash.  Most coax has little loss on 160.  I 
would recommend inexpensive RG213 for this.  However 9913 will be fine 
if you seal the connectors really well and keep the water out of it.

Lee said:  ... 50 feet of 9931 coax with a 6" PVC air choke made from 25 
feet of 9931 wrapped....

This should provide an impedance of about 365 ohms at 2 MHz if the balun 
is wound on a single layer.  That's not enough, assuming the balun you 
picked is really a voltage mode balun.  Before recommending a fix, lets 
examine what you have so far.

With a 260 ft dipole, 85 ft in the air and 65 ft of 600 ohm line, the 
impedance presented to the balun is about: 
at 1.8 MHz 126+j605
at 1.9 MHz 277 +j960. 

If the balun is perfect (and it won't be) the impedance presented to the 
9913 will be about:
at 1.8 MHz 10.4 +j50, SWR 9.7
at 1.9 MHz 22 +j78,  SWR 8.2

After 75 ft of 9913 (25 ft of which is the balun) the impedance 
presented to the rig is:
at 1.8 MHz 270 -j230, SWR 9.4
at 1.9 MHz 43 -j116, SWR 8.1 

EZNEC says the coax loss is 0.17 dB or about 60 watts at legal limit, 
not bad.  However you will need a good tuner with this because the SWR 
will be very high.

You didn't say if you were planning to use this on more than 160 
meters.  The only problem with this antenna on the higher bands is that 
the lobes become very narrow, and on some bands the azimuth coverage is 
not very good, however if a station happens to be in one of the lobes, 
the gain is good.  The SWR at the station end is not too bad either.  
160 is the worst band for SWR.  Coax loss is also low on all bands.  Be 
sure and have good insulation at the dipole feedpoint because the 
voltage at this point gets pretty high on some bands.

The numbers above could be somewhat in error because if the balun is a 
voltage mode balun, an insufficient choke will cause some common mode 
current on the feedline and change the SWR some.  Also the 12 to 1 balun 
has to operate with a relatively high reactance and it is unlikely that 
it will really look like a 12 to 1 in that situation.

Now to address the choke problem: The 16 turn choke should have about 
365 ohms at 2 MHz and be self resonant somewhere around 8 MHz assuming 
it is wound in one layer.  Lowering that resonant frequency will make 
the impedance on 160 go up, but it will make it go down on the higher 
bands.  So if you only want to use this antenna on 160 meters you could 
double the number of turns and get about 1000 ohms at 2 MHz and a self 
resonant frequency of about 5 MHz.  However if you are going to use this 
antenna on multiple bands, I would recommend a choke made from a ferrite 
toroid.  That will have a much greater bandwidth.

A better solution is to buy a balun that is a current mode balun.  With 
that you may not need an additional choke.  All the DX Engineering 
baluns are current mode unless specified otherwise.  Note: There isn't 
much SWR difference between using a 9 to 1 balun and a 12 to 1, a little 
lower on some bands, a little higher on others.

I hope this helps.

Jerry, K4SAV

Its from Onion wrote:

>Hello all,
>
>I am in the design stage of a dipole antenna for 160m and would like some professional input in case I missed something.  All input is appreciated and welcomed.  I'm a bi-lateral amputee so my time table is dependent on my friends time!  haha
>
>The jest of it:
>260 feet of #12 wire suspended at or about 85 feet.
>65 feet of 600 ohm ladder line feeding the wire directly.
>A mil-spec balun from Antenna Products rated at 2-30M 4Kw PEP
>http://www.antennaproducts.com/BINDEX.htm<about:blank> SM-153.
>50 feet of 9931 coax with a 6" PVC air choke made from 25 feet of 9931 wrapped. 
>This is tuned with an Ameritron ATR-15 "T" type tuner
>
>Legal limit when needed from a AL-1200 Ameritron.
>
>Will send a photo of the balun should you want to see.
>
>Thanks again and
>
>73,
>Lee
>KE4VYN
>www.qrz.com/callsign/ke4vyn<about:blank>
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