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The Force12 Flagpole and FP-40C under the microscope

The Force12 Flagpole is a ground mounted unloaded vertical of 4.7m height with four shallow-buried radials. The FP-40C is an optional base-loading coil.

Though the Force12 Flagpole is the subject of this article, the issues raised relate to the the basic configuration of a ground-mounted short vertical with radials with or without base-loading.

Manufacturers claims / info

Force12 Inc advertises 'Our flag pole antenna operates on all bands, 40 through 10 meters using your rig's tuner'.

Force12 Inc states 'Although the flagpole is efficient and works DX on 40, it can be significantly improved by base-loading' and offers the FP-40C base-loading coil for improved 40m efficiency.

Force12 Inc's web page (at 17/03/2007) suggests the feed-point impedance of the basic vertical at 7MHz is 5-j460Ω and VSWR=16. The impedance seems reasonable for the radiator itself, but almost certainly ignores the equivalent ground loss resistance. The VSWR would appear to be stated for the transmitter end of the transmission line and so understates the mismatch problem.

Force12 Inc describe the loading coil as 10μH with a Q of 600. The Q is very high, but for the purpose of this analysis, I use their figure. The claimed feed point Z is now 6-j22Ω and VSWR is 6. Again, the VSWR is the source end VSWR and so understates the mismatch problem.

Force12 Inc states that there is 16dB of loss on 100' of RG213 coax because of the mismatch of the unloaded vertical, and implies that the FP-40C addresses that loss, the reader might form the impression that it nearly eliminates the 16dB of loss.

Modelling

The antenna system is modelled here using the stated radiator impedance with three options for ground resistance, connected by 100' of RG213 as per Force12 Inc's web page, and a T-match ATU.

Ohmic loss on the radiator tube is ignored; it will be very low; so all of the 5Ω radiator resistance is taken as radiation resistance.

ATU loss is estimated using W9CF's T match applet with the default settings.

Transmission line solutions are from the RF Transmission Line Loss Calculator / Enhanced .

Table 1: Summary of data from modelling
Scenario Unloaded Loaded
Rground (Ω) 0 10 20 0 10 20
Radiator Z (Ω) 5-j460 5-j460 5-j460 5-j460 5-j460 5-j460
Loading coil Z (Ω) 0 0 0 0.7+j440 0.7+j440 0.7+j440
Ground Z (Ω) 0 10 20 0 10 20
Ground + loading coil loss (dB) 0 4.8 7.0 0.6 5.0 7.1
feed-point Z (Ω) 5-j460 15-j460 25-j460 5.7-j20 15.7-j20 25.7-j20
Load end VSWR 498 230 150 9.9 3.7 2.3
Line input Z (Ω) 8.7-j73 9.0-j73 9.5-j73 10.0+j5.0 16.2-j3.4 23.8+j0.46
Source end VSWR 17.0 16.3 15.7 6.4 3.1 2.1
Line loss (dB) 16.8 12.2 10.2 2.1 1.0 0.7
ATU loss (dB) 1.1 1.1 1.0 0.5 0.4 0.3
System loss (dB) 17.9 18.1 18.1 3.2 6.4 8.1
System efficiency (%) 1.6 1.5 1.5 48 23 15

Table 1 shows a summary of the data from modelling. The columns with Rground=0Ω reflect the manufacturer's perspective, but are unrealistic. The columns with Rground=10Ω are representative of a quite extensive ground system. W4RNL's article Models with Buried Radials: A Small Compendium suggests that under his modelled average ground conditions, it needs 16 radials to obtain around 10Ω of Rground; four radials (as supplied with the vertical) are more likely to deliver Rground closer to 20Ω.

Note the unloaded system efficiency of around 1.5%, which does not support the manufacturer's claim 'although the flagpole is efficient...'.

Note the complex interaction of system components. Looking at the first three data columns, though ground+loading loss increases by 7.0dB, system loss increases by only 0.2dB, being offset mainly by reduced transmission line loss, yet that doesn't happen in the following three columns.

While the manufacturer and product users claimed that the unloaded antenna 'works' , less than 1.6W is actually radiated from a 100W transmitter.

Fig 1: Comparison of losses Unloaded / Loaded

Fig 1 shows the losses for Rground=20Ω (which is probably typical of a minimal radial installation in average ground) for the loaded and unloaded cases. Clearly, the loading reduces system losses significantly.

It is popularly held that resonant antennas 'work much better', and that explanation might be offered by some for why this improvement occurs. In fact the loaded antenna is still off resonance, and increasing the loading coil to resonate it improves the transmission line and ATU losses by only 0.2dB. The reason the large improvement with loading occurs is that the loading reduces the line VSWR from 150 to 2.3, which reduces line loss from 10.2dB to 0.7dB, and ATU loss from 1.0dB to 0.3dB, yielding an overall improvement of 10dB.

The claimed coil Q of 600 is very high; it has extremely low losses. A coil of Q=200 has about 0.4dB worse coil loss in the Rground= 10Ω scenario and about 0.3dB worse coil loss in the Rground= 20Ω scenario, impact on total system loss is a little lower. Clearly, the Q of the supplied coil is much higher than needed for the purpose.

Improvements

The performance of the 4.7m high flagpole at 7MHz is dogged by its low radiation resistance relative to practical ground losses.  A 7.2m high flagpole (two more sections of 1.2m) has a radiation resistance around 15Ω which would (with an appropriate loading coil) in an Rground= 20Ω environment deliver ground+loading losses of 4.0dB (and slightly reduced feedline and ATU losses), about 3dB lower than the 4.7m antenna. You could view that as 3dB increase in gain for a 2.4m extension of the vertical.

Fig 2: Ground+loading loss for short base-loaded vertical at 7MHz.

The relationship between the losses of a short base-loaded ground mounted vertical and its height in practice is interesting; there is a rapid increase in loss below a critical height in a particular scenario. Fig 2 shows the modelled ground+loading loss for a short base-loaded vertical at 7MHz over average ground. Note that losses increase rapidly below about 7.2m or 17% of a wavelength.

For more information, see the article  An unloaded vertical as a multi-band HF antenna. This article explains that a 13m high vertical with an automatic ATU at the base will perform quite well down to 3.5MHz, and by scaling, it can be expected that 6.5m high vertical with an automatic ATU at the base will perform quite well down to 7MHz, or 4.7m (eg a standard Force12 flagpole) height down to 10MHz.

Conclusions

Force12 Inc's promotional material appears to ignore the effects of equivalent ground system resistance (loss) at the feed point, which leads to unrealistic predictions of the effect of the FP-40C coil, and over specification of Q (if indeed it is 600).

The unloaded flagpole has very low efficiency at 7MHz.

The FP-40C base-loading coil probably reduces system losses in the above scenario by about:

Lengthening the vertical reduces ground loss significantly (though the FP-40C loading coil is not appropriate to a longer vertical).

Ground system performance is critical to efficiency.

Outcomes are sensitive to the model scenario, particularly ground conditions and feed line type and length.

The analysis demonstrates that components of an antenna system interact with each other in a complex way, and it is important to analyse the entire antenna system (radiator, ground, transmission line, balun, ATU etc) to obtain a correct understanding of how the system works overall.

Links

Glossary

Term Meaning
ATU Antenna Tuning Unit - a network for transforming impedance
efficiency The ratio of power out to power in
dB decibel
decibel a power ratio expressed as 10*log(P1/P2)
Gain Directivity - loss
System efficiency For an antenna system, the ratio of power radiated to power from the transmitter
System loss For an antenna system, the inverse of system efficiency
VSWR Voltage Standing Wave Ratio
 

Changes

Version Date Description
1.01 16/03/2007 Initial.
1.02    
1.03    

V1.01 12/02/09 01:04:59 -0700 .

Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.


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