The unloaded Q of the inductor should have a very minor effect on the
bandwidth and efficiency of WX7G's 67 foot antenna on 160m. The series
resistance of a 70uH coil with Q=400 is only about 2 ohms. If the coil
Q was 600, the Rs would drop to about 1.34 ohms. The sum of the
radiation resistance and ground loss resistance would probably be much
higher and swamp such a small change in coil Rs. In addition, the
current in the coil is less than the current at the feedpoint, so this
transforms the coil loss Rs to a smaller value at the feedpoint.
My EZNEC model shows the feed Z goes from 23.74 ohms down to 23.38 ohms
if the top load Q rises from 400 to 600. The gain at 24 degree
elevation rises from 0.18 dBi to 0.25 dBi. The 2:1 bandwidth drops from
43 to 42 kHz. The top coil dissipation drops from 69 watts to 46 watts
(with 1500 watts drive). This assumes a fairly good ground system with
5 ohms of equivalent loss resistance (which appears in series with the
radiation resistance at the feedpoint) which burns up about 300 watts of RF.
A lot of commercial ham antenna manufacturers depend on high coil losses
to provide a match to 50 ohms at the feedpoint, especially in mobile
antennas. Of course, you pay for this in poor signals. Some mobile
loading coils are near self-resonance at the operating frequency, which
can really burn up RF due to circulating currents.
73, Terry N6RY
PS - Art, thanks for the link to the nice inductance calculator. Since
this is the Antennaware list, you need to get your feet wet with antenna
modeling, too. It's disgustingly addictive!
On 2010-01-31 7:34 PM, Art Trampler wrote:
> Dave,
>
> Sounds promising...and making more sense than my idea of using irrigation
> pipe.
>
> First off, since you obviously understand more about this than I do, thank
> you. Second, you might enjoy this calculator and discussion:
>
> http://hamwaves.com/antennas/inductance.html
>
> In order to minimize coil losses, would a more square design (length to
> diameter) be preferable? Or are you concerned about the coil having too
> great a Q and therefore limited bandwidth?
>
> In my "envisioning" of using the coil to choke the radiator around 65 feet
> or so, I was thinking of a large coil, perhaps 4 or 5 inch diameter. I run
> the Hy-Gain AV640 and notice the coils do warm up, so assume that such heat
> is loss.
>
> The 2" diameter coil would be about 8 inches long, right? This program
> shows a Q of about 400 ohms at 1.8mhz, which surprised me. Is there any
> advantage or disadvantage, electrically, to a coil that was similarly wound
> but 3.5" x 5"?
>
> Thank you for working this through...it sounds great and do-able. I am
> curious as to your thoughts on different coil design methodologies.
>
> Art
>
>
> -----Original Message-----
> From: antennaware-bounces@contesting.com
> [mailto:antennaware-bounces@contesting.com] On Behalf Of DAVID CUTHBERT
> Sent: Sunday, January 31, 2010 7:56 PM
> To: aa4nn
> Cc: antennaware@contesting.com
> Subject: Re: [Antennaware] center loading versus ground loading
>
> Gary,
>
> The Battlecreek is a good antenna. However, It looks like a lot of work
> though to get the tubing lengths correct before and after the 40 meter trap
> as well as to tune the 160 meter wire.
>
> Here is two band vertical that is easier to tune up. It is for 80 and 160
> meters and it tuned from the base. Put up the antenna one time and tune at
> the base. No relays. Direct feed with coax.
>
> We can write an article on the antenna for QST, CQ, or AntenneX if you'd
> like. Here is the antenna:
>
> *160 meters:*
> A 67' mast. This can be the long vertical kit from DX Engineering (about
> $200). On top is a top hat consisting of six 0.5" aluminum tubes 6' long.
> Right below the top hat is a 70 uH inductor. It is made of 2 inch PVC pipe
> with close wound #14 THHN wire (from the hardware store). To adjust it to
> resonance on 160 meters a 10 uH base coil is adjusted.
>
> The top hat mass can be reduced by using the DX Engineering top hat with the
> long spokes. The inductor will need to be sized for this. I can do this in
> EZNEC.
>
> *80 meters:*
> A 64' wire spaced 2' from the mast. It is base tuned with a loading coil.
> About 2 uH. The bottom of this coil connects to the bottom of the 160 meter
> tuning coil. The coax attaches there.
>
> Put up the vertical. Tune 160 meters. Tune 80 meters. Done.
>
> Dave WX7G
>
>
>
>
> On Sun, Jan 31, 2010 at 1:23 PM, aa4nn<aa4nn@earthlink.net> wrote:
>
>
>> HI Gary,
>> All you need do is emulate the Battle Creek Special.
>> You can google to get specifics. The BCS uses only
>> one wire to top load for 160m and only one wire to top
>> load for 80m. Excellent antenna...no switches, no tuning,
>> just change bands and go. If you are unable to find
>> specs, let me know.
>> 73& all the best.
>> de Joe, aa4nn
>> Lake Wylie, SC
>>
>> ----- Original Message -----
>> From: "Gary K9GS"<garyk9gs@wi.rr.com>
>> To:<antennaware@contesting.com>
>> Sent: Sunday, January 31, 2010 2:41 PM
>> Subject: Re: [Antennaware] center loading versus ground loading
>>
>>
>> I have been contemplating an antenna design to cover 40, 80, 160M.
>> Basically ~32 ft of aluminum tubing/mast, a trap, then more tubing/mast to
>> ~60 ft (possibly using a capacity hat to tune on 80M) and then a second
>> trap
>> above the capacity hat with a T-top loading wire to tune on 160M. My goal
>> would be no switching/control lines at the antenna. I work almost
>> exclusively CW so I don't need to cover the entire 80/160M band.
>>
>> Thoughts??
>>
>> 73,
>>
>> Gary K9GS
>> ++++++++++++++++++++++++++++++++++++++++++++++++
>> K9GS
>> Gary Schwartz email: k9gs (at) arrl.net
>> Check out K9NS on the web: http://www.k9ns.com
>> Society of Midwest Contesters (SMC) http://www.w9smc.com/
>> GMDXA http://www.GMDXA.org<http://www.gmdxa.org/>
>> ++++++++++++++++++++++++++++++++++++++++++++++++
>>
>> ----- Original Message -----
>> From: "Art Trampler"<atrampler@att.net>
>> To:<antennaware@contesting.com>
>> Sent: Friday, January 29, 2010 10:48 PM
>> Subject: [Antennaware] center loading versus ground loading
>>
>>
>> I live on a fairly small lot and use a Hy-Gain AV640 for 40 through 10 and
>> currently have no antenna for 80 or 160.
>>
>>
>>
>> My backyard is about 90 x 70 but has some ill-placed and ill-shaped trees
>> for either wire antennas or a tower.
>>
>>
>>
>> So I am thinking of another vertical, but this one ¼ wave with a radial
>> field. I would like to get 80 and 160 out of it. I don't mind having to
>> guy it, or even having to pour a concrete base for it.
>>
>>
>>
>> My first thought is to use aluminum irrigation pipe as others have, and
>> have
>> about 60 to 65 feet of it, an insulator and inductor and high voltage
>> relay,
>> and then perhaps 15 to 20 of much smaller aluminum tubing, with a sloping
>> capacity hat of four wires going partially down the four top guys. I
>>
> don't
>
>> know if I could get away from the relay, and put up a trap instead but am
>> wary of using a true trap (coil& capacitor) rather than just a large,
>>
> high
>
>> Q coil.
>>
>>
>>
>> As you can see this idea is full of possibilities and mechanical
>>
> drawbacks,
>
>> so the question is, is there that much to gain from the center-loaded
>> design
>> with capacity hat, versus a switchable tuning network at the base of the
>> antenna?
>>
>>
>>
>> Your input is appreciated. I am hoping to make this a summer project and
>> reward myself with 80 and 160 in the winter.
>>
>>
>>
>> 73,
>>
>> Art
>>
>>
>>
>> Art Trampler, KØRO
>>
>>
>>
>> _______________________________________________
>
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