[Elecraft] OT antenna question
Bob McGraw K4TAX
rmcgraw at blomand.net
Wed Jan 13 07:40:58 EST 2016
I'm always fascinated by those which use of a 4:1 balun when 450 ohm
line is used. In most all cases, the 450 ohm line is operated at a
known high SWR. The nature of 450 ohm line {or other line such as 300
ohm or 600 ohm, etc.} is that it exhibits much lower loss when operated
at a high SWR as compared to coax under the same conditions. The
point...... we are not matching the feed-line impedance. Even if we
desired to match the feed line, it would need to be terminated into a
450 load. Even then with a 4:1 balun we would have a ~2.25:1 SWR best
case at the source. [450/4=112.5/50=2.25]
The feed-line basically reflects the impedance of the antenna from the
feed point to the source. Although at certain electrical lengths of
line it can serve as a 1/4 wave transformer thus transforming the
antenna impedance to another value, we typically find that a 1:1 current
balun is better suited for the application when working between a
balanced line/load and a tuner.
So where does a 4:1 balun become useful? In general, any place the feed
point of the antenna presents a 200 ohm load. Example: a resonant
folded dipole that is less than 1/2 wavelength above the surface of the
earth is typical. In this example, a 4:1 balun at a 200 ohm load will
present a 50 ohm load to the source. [200/4=50]
Just remember that most tuners have their greatest loss when the load Z
is less than 50 ohms. Many tuners being required to match loads in the
5 to 10 ohm range can have as much as 50% to 75% power loss. This
manifests into heat dissipated in the tuner components and watts of RF
which does not get to the antenna.
The typical use of a 4:1 balun transformer action will divide the load
by a factor of 4. Of course one can use a 1:4 balun, in other words,
turn it around to gain some advantage in the impedance matching game.
Just be sure the balun can handle the voltages involved. This is a
point for another long discussion.
73
Bob, K4TAX
On 1/12/2016 10:58 PM, Joe Subich, W4TV wrote:
>
> On 1/12/2016 10:57 PM, Bob McGraw K4TAX wrote:
> > If your tuner uses disk ceramic caps, as many do, these can be
> > heating thus not being able to handle the RF current. They heat
> > and cool and change value which in turn changes tuning.
>
> Particularly if you are using a 4:1 balun! On 40 meters the impedance
> of the half size antenna is *very low*. The 4:1 balun will further
> decrease the impedance seen by the tuner causing it to work at its
> absolute worst efficiency (highest circulating current).
>
> At the very least see if you can extend the dipole to 44', then get
> some Smith Chart software and see if you can find a compromise
> feedline length that provides a reasonable impedance on all bands,
> and finally replace the 4:1 balun with a high quality 1:1 current
> balun.
>
> By extending the dipole you should be able to find a compromise
> feedline length that will provide a 100 - 300 Ohm impedance on
> all bands - something the tuner will be much happier to with and
> be far more efficient than working at best than 10 Ohms as it
> probably is doing on 40 meters.
>
> 73,
>
> ... Joe, W4TV
>
>
> On 1/12/2016 10:57 PM, Bob McGraw K4TAX wrote:
>> If your tuner uses disk ceramic caps, as many do, these can be
>> heating thus not being able to handle the RF current. They heat and
>> cool and change value which in turn changes tuning.
>>
>> Replacing them with suitable RF current rated units is the solution.
>>
>> Bob, K4TAX
>>
>>
>> Sent from my iPhone
>>
>>> On Jan 12, 2016, at 5:27 PM, Guy Olinger K2AV <k2av.guy at gmail.com>
>>> wrote:
>>>
>>> Hi Vic,
>>>
>>> Your story does suggest trouble at very high current points. Just a
>>> list of
>>> things below I've heard or seen to stir up a new idea or two. No
>>> opinion on
>>> which if any makes any sense in your situation.
>>>
>>> ---------
>>>
>>> Insect nests in tubular spacers on open wire.
>>>
>>> Spacers have carbon tracks.
>>>
>>> Material inside the balun housing is much hotter than touchable
>>> material.
>>>
>>> Stranded wire in open wire has been waterlogged, corroded and the
>>> remaining
>>> conductor material at a current max along the line is heating up.
>>>
>>> Wire inside insulation is nearly all broken, or is broken, and
>>> contact is
>>> miscellaneous and highly resistive.
>>>
>>> Insulation on the wire is significantly compromised by ultra violet or
>>> critter nibbling, letting in water to the stranded conductor.
>>> Advice, has
>>> been to use bare solid #12 copper or larger for open wire that is
>>> carrying
>>> large standing waves, to handle the current maximums.
>>>
>>> Check heat all around the entire core. I have burned up some number of
>>> cores before I understood the materials and engineering. On one all the
>>> damage was on a spot that comprised only 15 degrees of the
>>> circumference.
>>>
>>> Electrical connections in aluminum elements made of telescoping
>>> tubing go
>>> highly resistive as water is boiled out of the joints.
>>>
>>> Connections made of dissimilar metals/materials go bad and become more
>>> resistive as water is boiled out of the joint.
>>>
>>> Tuner rotary switches going bad.
>>>
>>> The rolling contact on on a variable coil losing its tension
>>> (numerous ways
>>> for this to happen depending on construction), contact is very small
>>> and
>>> becomes worse as it heats up. Fixed by repairing mechanism for
>>> maintaining
>>> contact.
>>>
>>> Parted conductors at conductor joints due to metal erosion at the
>>> contact
>>> point.
>>>
>>> --------
>>>
>>> Your troubles would have me taking down the dipole, rebuilding it
>>> with all
>>> mating surfaces wire-brushed and reassembled with clear silicone
>>> dielectric
>>> grease, completely new hardware likewise treated, with all balanced
>>> conductors replaced new, using bare solid #12 outdoors, and brand
>>> new runs
>>> indoors. Old wire to recycling.
>>>
>>> Hope this has kicked off a new idea for you. 73 and good luck. Guy
>>> K2AV
>>>
>>> On Tue, Jan 12, 2016 at 4:24 PM, Vic Rosenthal 4X6GP/K2VCO <
>>> k2vco.vic at gmail.com> wrote:
>>>
>>>> I'm having a problem which has me stumped. I'm going to describe my
>>>> complete antenna and feed system because something in it is
>>>> misbehaving and
>>>> I don't know what!
>>>>
>>>> My system works on all bands from 40 to 10 (or it should).
>>>>
>>>> The antenna is a full-size 20m rotary dipole. It is all aluminum
>>>> tubing,
>>>> no traps or stubs. Just a dipole. I am feeding it with about 30
>>>> feet of
>>>> "true ladder line," which is open wire line made of #16 insulated wire
>>>> spaced about 3-1/4" with black PVC spacers every 18" or so, except
>>>> near the
>>>> antenna and the rotor where I've added extra ones so that the spacing
>>>> doesn't change when the antenna rotates.
>>>>
>>>> The line comes into the shack and is connected to a static drain,
>>>> which is
>>>> a box with two 10-megohm high voltage resistors to ground and a
>>>> couple of
>>>> spark gaps. Then a piece of 450-ohm window line about 3 feet long
>>>> connects
>>>> it to a pair of large air variable capacitors in series with each
>>>> leg which
>>>> knock out some of the reactance on 40m to make it possible to tune
>>>> more
>>>> easily. Then a very short piece of window line connects to a big
>>>> 5kW DX
>>>> Engineering 4:1 balun, spec'ed for tuner service, and finally via a
>>>> piece
>>>> of RG-213 18" long, to a T-network tuner.
>>>>
>>>> My K3 drives a TL922 amp and I have an SWR meter in line.
>>>>
>>>> Now here is my problem: it works OK on all bands except 40 meters.
>>>> On 40,
>>>> it tunes up fine with low power, but when I run more than a couple of
>>>> hundred watts, after perhaps 10 seconds of key-down, the SWR starts to
>>>> climb. I have watched it go to 4:1 before I stop sending for fear of
>>>> destroying something.
>>>>
>>>> The SWR rises both on the meter in the tuner and the extra one I
>>>> have in
>>>> line.
>>>>
>>>> Classic symptoms of something heating up. But what?
>>>>
>>>> - The tuner components are all cold.
>>>> - The coax to the balun and its connectors are cold.
>>>> - The balun itself is just barely perceptibly warmer (I have to
>>>> touch the
>>>> core to tell).
>>>> - The window line, the static drain resistors, the air capacitors
>>>> and all
>>>> the connections in the shack are cold.
>>>>
>>>> I know the SWR is astronomical on 40 meters, so currents and
>>>> voltages are
>>>> high. But nothing in the shack seems to be heating up. Any more
>>>> ideas of
>>>> where to look?
>>>>
>>>> --
>>>> 73,
>>>> Vic, 4X6GP/K2VCO
>>>> Rehovot, Israel
>>>> http://www.qsl.net/k2vco/
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