Maybe some useful hints in the instructions for a quick 20m GP antenna kit that SOTAbeams once provided.
73, Colin
I often use this with my EFHW and it avoids the CM RF-pickup related issues.
However, I note that whilst the SWR meter is happy, the RF power meter on the radio often reads as little as half of it’s usual reading (eg as seen without the choke) with the ugly choke in place.
Have some ideas as to why, but would appreciate explanations from an informed expert.
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It only works with “full fat” Coke.
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I put pictures just to show that gp i working. Radiator and radials are resonating in the band. Antena is low above ground and is working. SWR is good and no rf isuess.
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hi Matt,
the output power indication is based on the incident voltage at the antenna connector. If the impedance presented by the antenna system is higher than 50 ohms or is reactive, the incident voltage will rise, even though the power is still the same. And the corresponding situation for low impedances.
As usual this is my understanding of it, I will leave it to others to offer more sophisticated answers.
73
Andrew VK1DA
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So based on that, the fact that inserting the choke lowers the reading of the radio’s power output meter implies that either:
- The CM choke is changing the complex impedance of the feed+antenna system to the differential mode signal, as seen at the antenna port (despite being supposedly transparent to DM signals) - thus lowering the voltage at the measurement point; or
- The reflected common mode signal is increasing the voltage at the antenna connector.
Which were the two possibilities that instinct had suggested to me.
Either way, does that imply that the power reading is spurious, and does not actually imply a change to the actual radiated power?
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When you have common mode current, it basically means that the feedline, and everything attached to it (which may include power cables, headphone cables and possibly the operator), becomes part of the antenna. An effective choke limits the RF to just the intentional antenna.
So it isn’t unexpected that the impedance seen by the radio changes when you add a choke - you’ve changed the antenna, after all. That affects the SWR and the output power from the radio. Note that the SWR may be either higher or lower: it isn’t uncommon to tune an antenna for low SWR, add a choke, and discover that the SWR is higher, because the original tuning accounted for that extra wire hanging on one side of the feedpoint.
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Well said!
73 Chris
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