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Author Topic: Screwdriver Ant minus Toroid  (Read 556 times)
N6JSX
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Posts: 274




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« on: August 27, 2019, 02:43:02 PM »

What HF Screwdriver antenna(s) do NOT use a 'Toroid' in the RF/coax line or match system?  Huh
[Beware some antenna mfg skillfully hide the Toroid inside the large coil mounting tube.]

Why? Toroid is used to fake the transmitter into believing the antenna is a good 50 ohm reactance to drive the radio to max RF output. The Toroid consumes energy in its iron-type core to accomplish this match robbing the actual antenna from TX energy. Having a Toroid in the screwdriver will give you a false antenna VSWR/resonance. Screwdriver is already compromised in length using the large coil (and capacitance) to compensate; adding a Toroid just further compromises a questionably resonate antenna.   Roll Eyes     
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K0UA
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Posts: 4837




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« Reply #1 on: August 27, 2019, 02:47:37 PM »

What HF Screwdriver antenna(s) do NOT use a 'Toroid' in the RF/coax line or match system?  Huh
[Beware some antenna mfg skillfully hide the Toroid inside the large coil mounting tube.]

Why? Toroid is used to fake the transmitter into believing the antenna is a good 50 ohm reactance to drive the radio to max RF output. The Toroid consumes energy in its iron-type core to accomplish this match robbing the actual antenna from TX energy. Having a Toroid in the screwdriver will give you a false antenna VSWR/resonance. Screwdriver is already compromised in length using the large coil (and capacitance) to compensate; adding a Toroid just further compromises a questionably resonate antenna.   Roll Eyes     

That isn't even remotely true.  The clamp on ferrite is to prevent RF from going back down the control cable. It hasn't a thing to do with resonating the antenna.  And what is questionable about a mobile antenna? Everyone knows their efficiency is very low and the lower in frequency you go or the shorter the antenna is the less efficient it is.
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73  James K0UA
ARRL Missouri Technical Specialist
N6JSX
Member

Posts: 274




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« Reply #2 on: August 27, 2019, 09:15:53 PM »

Now you know why I'm not an ARRL member - with tech-info like this from ARRL guru's that must comment on everything ?!?  Cry
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W9IQ
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Posts: 3563




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« Reply #3 on: August 28, 2019, 04:15:15 AM »

Toroid is used to fake the transmitter into believing the antenna is a good 50 ohm reactance to drive the radio to max RF output. The Toroid consumes energy in its iron-type core to accomplish this match robbing the actual antenna from TX energy. Having a Toroid in the screwdriver will give you a false antenna VSWR/resonance. Screwdriver is already compromised in length using the large coil (and capacitance) to compensate; adding a Toroid just further compromises a questionably resonate antenna.   Roll Eyes     

It seems you are misunderstanding some of the terminology and what is happening. The comments from James are completely accurate - your insult is unwarranted.

Firstly, you never want an antenna to appear as 50 ohms of reactance. The goal is 50 ohms of resistance. If it were purely reactive the antenna would not radiate at all and your transmitter would either preventively shutdown or it would burn out the finals.

Secondly, most toroids used by hams are now made of ceramic materials. While iron toroids are still available and find some uses, their usable frequency range and their low permeability limit their applications.

When a coaxial cable or a control line is wrapped around a toroid of suitable material, it forms a choke that impedes the flow of RF current on the wire. When used with a coaxial cable, this choke is commonly called a choking balun or a current balun. This also prevents the exterior shield of the coax from becoming part of the radiating antenna.

The current balun does not rob the antenna of RF energy. It essentially does the opposite by not allowing (or at least greatly reducing) unwanted RF energy from flowing on the exterior of the coax shield.

The toroid does not play a direct role in "resonating" the antenna. However, in the absence of a proper current balun, the exterior shield of the coax becomes part of the antenna. Now the screwdriver inductor must attempt to match the antenna with the added shield length. This is usually not intended by the screwdriver antenna designer so the matching range may change.

The summary is that the toroid choke/balun used with a screwdriver antenna keeps undesirable common mode current off of the exterior of the coax shield and off of the control lines. These chokes have no negative effect on the performance of the antenna. If you find a screwdriver antenna that is supplied without these chokes, it would be wise to add them to the installation.

You can learn more about toroid based baluns and chokes in the ARRL Handbook, the ARRL Antenna book, and the ARRL RFI book. You do not need to be a member to buy or read these books.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
K0UA
Member

Posts: 4837




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« Reply #4 on: August 28, 2019, 07:36:43 AM »

Now you know why I'm not an ARRL member - with tech-info like this from ARRL guru's that must comment on everything ?!?  Cry

 Why did you post in a public forum in the first place if you did not want your views to be debated?  And what does the ARRL have to do with it?
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73  James K0UA
ARRL Missouri Technical Specialist
M0GVZ
Member

Posts: 403




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« Reply #5 on: August 31, 2019, 03:41:06 PM »

Now you know why I'm not an ARRL member - with tech-info like this from ARRL guru's that must comment on everything ?!?  Cry

Sorry but he is right and you're talking complete rubbish.
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WB6BYU
Member

Posts: 18535




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« Reply #6 on: August 31, 2019, 05:38:08 PM »

Note that there are two different uses for toroids at the base of mobile antennas.

One is as a common mode choke, as others have commented on.

The second is as a matching transformer to step up the low impedance of the antenna to 50 ohms.
This isn't necessarily required, if you don't mind the antenna being 15 - 35 ohms or so.  You can
also use a shunt capacitor or coil (which can be a toroid or solenoid form) connected across the
feedpoint to raise the impedance, but the value needs to be changed for each band.

The toroid matching transformer, if properly designed, should have low losses, so dissipate little
power.   You can check this by using the antenna for a while then testing the temperature of the
coil with your "digital temperature sensor" (aka "finger").  If it is lossy, then it will get warm,
because the lost power is transformed into heat.

You can also just feed the antenna with coax and use an autotuner at the rig, and the overall losses
probably won't be too high.  But the transformer approach gives a better match to 50 ohms across
the operating range of the antenna, without requiring manual intervention when changing bands,
which is the purpose of the screwdriver antenna in the first place.


Note that the science of core and transformer design has advanced greatly since the time I got my
license.  Certainly I've seen some projects that choose the wrong core materials, leading to high
losses.  But with proper choice of ferrite or powdered iron, and the right chore characteristics,
modern toroid transformers need not be lossy.

After all, a standard AC power transformer uses an iron core to match impedance (that is, higher or
lower voltage at lower or higher current, respectively) and if those were lossy the equipment would
heat up a lot more than it does.  The same goes for RF.
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G4ZOW
Member

Posts: 119




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« Reply #7 on: September 30, 2019, 08:46:50 PM »

Conor,

You state "Sorry but he is right and you're talking complete rubbish."

Yet on QRZ.com you state "I have been licenced since 2009 and am still learning the hobby."

IMHO Glenn is correct and explains this far better than I could.

I have found out what works and what does not through a lot of personal experimenting over many years of solely mobile operating. That torroid does not consume power and a Bird 4410 inline power meter as well as a remotely located spectrum analyzer testify and I tried with and without back in the 80's with both a monobander and Screwdriver antenna.

David G4ZOW/5B4 mobile

212 countries mobile to mobile including  50+DC US states confirmed.





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