[TowerTalk] HF LOG-PERIODIC ANTENNAS Comments Please

[Larry Phipps]

I used a 3 element model in EZNEC... I'm pretty sure there's a model of stacked 3 el SteppIRs on my website at www.telepostinc.com/n8lp.html in the downloads section. You can delete the wires for the second beam. I assumed 100' of RG-8, and used TLW to calculate loss. I added out-of-band feedpoint mismatch loss and antenna response as well.

My assumptions about distortion numbers were made with regard to a preamp stage only with a NF of 10dB and IP3 of 10dB. I did not include any preselector attenuation, which in some rigs might be substantial. It might be that there would be enough attenuation in the preselector that the difference between a yagi and LPDA would be inconsequential, with distortion products being below the noise floor in both cases. My main concern would actually be the blocking dynamic range spec of the rig, and nearby big signals. The use of LPDAs in a multi-multi environment, for instance, might require a more judicial use of traps, bandpass filters or external pre-selectors to block out strong local interference to reduce de-sensitization.

Larry N8LP 


----- Original Message ----- 
From: Jim Lux 
To: Larry Phipps ; Towertalk at contesting.com 
Sent: Wednesday, June 23, 2004 10:29 AM
Subject: Re: [TowerTalk] HF LOG-PERIODIC ANTENNAS Comments Please


Could you publish the model you used, including the length and type of
feedline you assumed?  I'm assuming that you're talking about out-of -band
mismatch loss?



----- Original Message -----
From: "Larry Phipps" <larry at telepostinc.com>
To: <Towertalk at contesting.com>
Sent: Wednesday, June 23, 2004 12:28 AM
Subject: Re: [TowerTalk] HF LOG-PERIODIC ANTENNAS Comments Please



I just ran a quick sweep of a 3el SteppIR model and the loss in the feedline
alone due to mismatch is about 4dB at 1 mHz out and 10dB at 5 mHz out. Add
to that the pattern loss and you're well over 10dB total out-of-band loss
compared to a LPDA. That might not seem like much, but 10dB lower
interfering signal is roughly 20dB lower distortion products. It all adds
up.






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