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Author Topic: Loading the 43' vertical for 160M  (Read 3087 times)
K3VAT
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Posts: 701




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« on: June 28, 2013, 05:08:33 PM »

I have a 43' aluminum vertical (DX Engineering) with twenty-five 70' and about ten 25' ground radials (the ten short radials run up against a wooden barn).  I would like to use this antenna on 160M.

I recently built the largest symmetrical capacity hat that I can: 4 wires each ~ 12 ft in length and sloping downward at ~ 45 degs.  I also have a fairly large diameter (3") loading coil inside a sealed 4" wide X 6.5" PVC enclosure (so I don't know how many turns nor the wire size).

When I test my setup (loading coil + 43' vertical + top hat) the resonant frequency is 2.35 mhz.  The bandwidth is very narrow.

I'd like to add a 2nd base loaded coil (also ~3" in diameter), but one that I can easily add & remove turns of #14 gauge wire to achieve a resonance = 1.83 mhz.  Then I'll work on the 50 ohm matching.  Note: I may be able to add a few feet of additional capacity hat to one pair of the wires, but I'd still would require considerable additional loading.

QUESTION: What would be the best mounting position for the new coil?  Can it simply mount directly above the existing coil, in the same axis?  At right angles? Does it have to be a certain distance away from the existing coil?  Any insights appreciated.

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K4SAV
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« Reply #1 on: June 28, 2013, 06:17:44 PM »

I would just wind a single new coil to do this.  Save the one you have for something else.  I figure the coil you have now must be about 18 uH, and you need about 32 uH.  The difference between 14 uH and 32 uH is only a few turns when it is only one coil.

Are you planning on running high power?  If so, corona may be a problem as the voltage at the base of the antenna will be very high.

Jerry, K4SAV
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WX7G
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« Reply #2 on: June 28, 2013, 06:29:47 PM »

I built a similar top loaded 160 meter vertical (see AntenneX magazine, The Big Vertical Project). It uses top loading and this will just about triple the radiation resistance. The final 10% of loading is accomplished at the base of the antenna. The base inductor is used to tune the antenna to resonance and to tap the coax center conductor up the inductor for a 50 ohm match.

If you are interested in doing this I'll model it unless Jerry can do it. The top coil is wound with close spaced #14 THHN stranded wire on 2" PVC pipe.

For the base coil I wind #14 bare wire on a 2" PVC pipe. The turns are spaced 0.25" so an alligator clip can be used. I place thick double stick tape on the PVC in three strips and that holds the wire in place.  



« Last Edit: June 28, 2013, 06:31:58 PM by WX7G » Logged
K3VAT
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Posts: 701




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« Reply #3 on: June 28, 2013, 07:33:06 PM »

I would just wind a single new coil to do this.
...
Jerry, K4SAV

Thanks Jerry, that was an option that I considered.

...
Are you planning on running high power? 
...

No, 200 watts; mostly CW

I built a similar top loaded 160 meter vertical (see AntenneX magazine, The Big Vertical Project).
...

Thanks Dave,  I only have a guest account for the AntenneX website and was unable to locate that article.

... It uses top loading and this will just about triple the radiation resistance. The final 10% of loading is accomplished at the base of the antenna. The base inductor is used to tune the antenna to resonance and to tap the coax center conductor up the inductor for a 50 ohm match.

If you are interested in doing this I'll model it unless Jerry can do it. The top coil is wound with close spaced #14 THHN stranded wire on 2" PVC pipe.
...

Yes, I'd be interested in this!  How how far up the vertical is this coil mounted?  I understood that 1) coils placed far up the vertical (>~ 80%) often require large amounts of inductance (sometimes approaching 200uH) and that required lots of wire with an increase in coil loss, and 2) for lowband operation in certain applications that using long coil form factors with closely spaced wire resulted in compromises in coil efficiency; so I didn't think that this was a viable way to go. 

73, Rich, K3VAT


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WX7G
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« Reply #4 on: June 30, 2013, 06:57:21 AM »

A NEC simulation with your antenna having a loading coil at the very top shows 74 uH will resonate it at 1.8 MHz. I placed my loading coil at the top for both electrical and mechanical reasons.

The radiation resistance of this is 8 ohms with uniform current along the antenna length. Base loaded with 36 uH the radiation resistance is 4 ohms.

Let's run some efficiency numbers with the base-referred ground loss being 10 ohms and the loading inductor Q being 200. The VSWR bandwidth is with the vertical matched to 50 ohms through a transformer (that's what I have used the past couple of years)

Top Loaded efficiency = 36%
Rr       8 ohms
Rgnd  10 ohms
Rcoil    4 ohms
2:1 VSWR Bandwidth  23 kHz
3:1 VSWR Bandwidth  38 kHz


Base loaded efficiency = 25%
Rr       4 ohms
Rgnd   10 ohms
Rcoil     2 ohms
VSWR bandwidth
2:1 VSWR Bandwidth 40 kHz
3:1 VSWR Bandwidth 66 kHz

« Last Edit: June 30, 2013, 07:04:11 AM by WX7G » Logged
K3VAT
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« Reply #5 on: June 30, 2013, 08:05:30 AM »

Thank you Dave !!  Early this morning I did send you an email about a few very specific points.  I will continue to study up on these suggestions.  73, Rich, K3VAT
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RFRY
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« Reply #6 on: June 30, 2013, 08:57:05 AM »

I'm getting different answers for the radiation resistance/system efficiency of a base-loaded 160m vertical using equations from Reference Data for Radio Engineers (6th edition), the 2 ohm coil loss, and the 10 ohm ground loss reported for the NEC model (see link below).

Also note that the base voltage for 100 W flowing into the base of the 43' vertical is over 1400 V (with no AM).

http://i62.photobucket.com/albums/h85/rfry-100/43ftVerton160m_zpsdaf87622.gif
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K3VAT
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Posts: 701




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« Reply #7 on: June 30, 2013, 01:05:18 PM »

I'm getting different answers for the radiation resistance/system efficiency of a base-loaded 160m vertical using equations from Reference Data for Radio Engineers (6th edition), the 2 ohm coil loss, and the 10 ohm ground loss reported for the NEC model (see link below).

Also note that the base voltage for 100 W flowing into the base of the 43' vertical is over 1400 V (with no AM).

http://i62.photobucket.com/albums/h85/rfry-100/43ftVerton160m_zpsdaf87622.gif

Thank you Mr Fry,

One possible explanation is that in your model you specified an antenna diameter of 2.5 inches; I'm assuming that this program will base its calculations on 'the entire antenna being 2.5 inches'.  But in fact the antenna is aggressively tapered from 2.125 to 0.375 in 15 segments, with perhaps, an overall average diameter of ~1 inch.  I believe (but not sure) that WX7G's model takes into account the tapered factor.  Could you rerun your analysis with an antenna diameter of, say 1.5", then 1.0" and if there is any noticeable difference (from the initial model) please post your results.  Thanks!

73, Rich, K3VAT
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WB6BYU
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« Reply #8 on: June 30, 2013, 01:22:26 PM »

I think that WX7G included the top hat wires discussed previously, which will increase
the efficiency.
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RFRY
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« Reply #9 on: June 30, 2013, 01:47:34 PM »

My analysis was made for a base-loaded (only) 43' vertical with no top loading wires/hat.  Maybe that was not consistent with the NEC model.

Here are the numbers from my spreadsheet for that same configuration except using a 1.5" and then 1" constant OD vertical conductor:

Rr essentially is the same for 2.5, 1.5, and 1" OD conductors, at 2.656 ohms.

Xc at 1830 kHz was 604 ohms for the 1.5", and 648 ohms for the 1" OD conductor.  The % change in this value across a given r-f bandwidth becomes greater with increasing Xc of the radiator at the center frequency  -- which means that, other things equal, the 3 dB r-f bandwidths of the smaller OD conductors become smaller.
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WX7G
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« Reply #10 on: June 30, 2013, 04:53:38 PM »

Yes my NEC model has the top loading wires.

I modeled the 43' tube as 1.5" diameter and the top wires as #14 wires. An error in these diameters will change the required loading inductance but not significantly change the radiation resistance. This simulation is dominated by ground loss resistance. Run the numbers for ground resistance of 5 ohms at the bottom end to 20 ohms at the top.

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K4SAV
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« Reply #11 on: June 30, 2013, 07:30:18 PM »

EZNEC calculations are tricky for this antenna.  The radiation resistance is very low and the efficiency depends highly on that number.  A small error in calculating radiation resistance results in a big change in efficiency.  I may as well give you the results of my calculations.

ANTENNA MODEL
43 ft vertical, 15 sections tapering from 2.15 inches to 0.375 inches
top hat 4 - 12 ft wires sloping at 45 degrees, no ring connecting outer ends
ground loss assumed to be 10 ohms, Mininec average ground

COIL
Located at 43 ft level
Dia = 3.5 inches (a 3 inch PVC pipe OD)
Length = 4.6 inches
T = 40 turns spaced at 2 wire diameters #16 wire
L = 85 uH measured at a low frequency
C = distributed capacitance = 4.2 pf
Fo = self resonant frequency = 8.7 MHz
Loss resistance 1.7 ohms

RESULTS
Antenna resonant frequency = 1.83 MHz
Antenna radiation resistance = 7.8 ohms
Feedpoint impedance before matching = 19.6 ohms

Including 10 ohms ground loss and 1.7 ohms coil loss, with 100 watts applied:
Ground dissipation = 51 watts
Coil dissipation = 12.6 watts
Antenna efficiency = 36.3%.  
Antenna gain = -2.3 dBi
2 to 1 SWR bandwidth after matching = 22 kHz

However, placing the coil at the top, actually surrounded by the top hat wires will increase the coil distributed capacitance resulting in increased dissipation in the coil.  The calculated coil loss of 1.7 ohms is probably significantly optimistic.  That will also move the resonant frequency lower.  I didn't think EZNEC could handle a helix model of the coil in this situation, so I didn't model it as such to evaluate the effect.  

I would suggest that all the inductance not be placed at the top.  Use a little less than required for resonance and tune it to resonance with a small coil at the base.  A network will be required at the base to match 50 ohms anyway.

-----
FOR BASE LOADED, no coil at the top:

COIL
33 uH
Dia = 3.5 inches (a 3 inch PVC pipe OD)
Length = 2.35 inches
T = 21 turns spaced at 2 wire diameters #16 wire
L = 33 uH measured at low frequency
C = distributed capacitance = 4.3 pf
Fo = self resonant frequency = 13.2 MHz
Coil loss resistance = 0.8 ohms.

RESULTS
Antenna resonant frequency = 1.82 MHz
Radiation resistance = 4.3 ohms
Feedpoint impedance before matching = 15.2 ohms
Applied power = 100 watts
Ground dissipation = 66 watts
Coil dissipation = 5.5 watts
Antenna efficiency = 28.5%
Antenna gain = -3.8 dBi
2 to 1 SWR bandwidth after matching = 40 kHz

A COUPLE OF OBSERVATIONS
Notice that with top loading the efficiency is about 36.3% and the gain is -2.3 dBi.  (Efficiency is probably optomistic because of coil proximity effects.) With base loading the efficiency is 28.5% and the gain is -3.8 dBi.  Although the efficiency, which is calculated in the near field, affects the far field gain, you can't calculate one from the other.  The current distribution on the vertical wire, which is different for these two configurations, changes the far field gain.

Notice that the bandwidth for the base loaded antenna is twice as wide as for the top loaded antenna.

Jerry, K4SAV
« Last Edit: June 30, 2013, 07:33:29 PM by K4SAV » Logged
NA0AA
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« Reply #12 on: July 01, 2013, 09:06:23 AM »

So, if I understand it, you have four individual wires from the top as your top loading - logical since each has a guy wire - could you connect the ENDS of each of the top hat wires, then in the middle of each cross connect, run another set of four, giving your eight interconnected top had wires and a hoop around the bottom?  That will greatly increase the top loading, the most efficient kind.

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K3VAT
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Posts: 701




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« Reply #13 on: July 01, 2013, 03:55:53 PM »

EZNEC calculations are tricky for this antenna.  The radiation resistance is very low and the efficiency depends highly on that number.  A small error in calculating radiation resistance results in a big change in efficiency.  I may as well give you the results of my calculations.
...
A COUPLE OF OBSERVATIONS
Notice that with top loading the efficiency is about 36.3% and the gain is -2.3 dBi.  (Efficiency is probably optomistic because of coil proximity effects.) With base loading the efficiency is 28.5% and the gain is -3.8 dBi.  Although the efficiency, which is calculated in the near field, affects the far field gain, you can't calculate one from the other.  The current distribution on the vertical wire, which is different for these two configurations, changes the far field gain.

Notice that the bandwidth for the base loaded antenna is twice as wide as for the top loaded antenna.

Jerry, K4SAV

Thanks Jerry for the model !!  Interesting numbers.  Because of the narrow bandwidth and low system efficiency in both models I'm now considering going to a substantially taller vertical.  My goals for this antenna system were to increase my DXCC totals from 110 (current) towards 200 over the next several years.  I also want to be able to boast my power capability to 500+ watts - I need to be heard.

73, Rich, K3VAT
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KB6HRT
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« Reply #14 on: July 01, 2013, 10:31:39 PM »

This is what I did, have been using it about 3 years an it works great, have a 43' vertical and a screwdriver mobil antenna in series with it, when to the top of the screwdriver antenna with breaded ground wire and clamped the brand above 3' above insulator then used a 15 turn shunt coil to ground at the base of the antenna. Can tune all of 160m-80-75m 20m-10m just with the screwdriver to a 1.3 to 1 SWR and using the tuner its flat an will take 1000w. Did use a ground plate and covered my yard with Nr 14 awg radials used about 3000' or wire some are as long as 66' others are as short as 18' the antenna tunes easy an works very well, by changing the shunt coil to ground you can get any band not covered with the 15 coil shunt the way I have it set up. you can get 40m or 17m. I get 60m flat with the screwdriver all the way down an its works well ............kb6hrt
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