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I have an Alpha Delta DX-EE antenna in my attic. The antenna is 40 ft long and my attic is about 5ft high by 55 ft long. There's a lot of other stuff up there--a chimney, Romex, Cat 6 cable, RG-6 cable, a TV antenna, junction boxes, etc. It tried to place the antenna so anything metal was at least a foot away. I ran about 60 ft of LMR400 up to the attic (I soldered N connectors on it).

I borrowed a friend's brand new Icom IC-7300. He hadn't tried it before. I connected the chassis ground to the ground prong on an electrical outlet and connected the antenna to the radio with a PL259 to N adapter. We didn't have much success receiving any stations on any bands. I'm in the Pacific Northwest and the band conditions were roughly same as pictured below:

band conditions

The Icom IC-7300 has a feature where it can make a bar-chart SWR plot. It transmits at 30W when doing so. I tried 10M and a believe a few other bands. No matter which frequencies I picked, all the bars in the chart were always pegged at an SWR of 3:1.

So I recently tested my antenna (and feedline) with a VNA. I only measured reflection on one port, however. These are my results (please ignore the phase plot--I got the feedline length compensation factor wrong):

return loss

I've marked the points with the lowest VSWR. The numbers are even better than in the review article I linked to above.

Why are my VSWR results so different between the radio and the VNA? Could a bad adapter cause this? I used a different N adapter for the VNA as opposed to the radio. Could this difference have anything to do with the radio operating at 30W and the VNA at -14 dBm?

Edit - 23-Mar-2019

After reviewing K1LI's answer, here are zoomed in versions of the plot above that show his point. The house is a split-level with two stories at one end of the antenna and one story at the other. On the two-story end, the antenna is about 21 ft AGL.

40m 20m 15m 10m

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  • $\begingroup$ I suspect that the IC-7300 is unable to measure an SWR greater than 3:1, and so what it's trying to tell you is that the internal antenna tuner's match is bad on all bands. If so, then the IC-7300 is actually agreeing with the VNA. You might try going to a county park or state park, throwing some ropes up in a couple trees (at least 20' / 6 m or so), and putting the antenna up temporarily; I bet the IC-7300 would be able to tune the antenna to a 1:1 SWR there. $\endgroup$
    – rclocher3
    Apr 23, 2019 at 15:13

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With the exception of 10 meters, the frequencies of lowest return loss (and SWR) measured with the VNA are below the ham bands on which you must have tested with the IC-7300. It's not easy to see on the graph, but the return loss within the ham bands appears to be about -5dB, which corresponds to an SWR of over 3:1. It's important to compare apples to apples when performing measurements; i.e., SWR should be reported on the same frequencies.

The antenna's resonant frequencies may be lower than expected because: it's close to the ground (you didn't say how high the attic is); it is being de-tuned by proximity to wiring and attic clutter; it is cut longer than required to permit tuning to higher frequencies by pruning; all of the above.

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