Thank you for the detailed response!
I’ll give this topic a go. What you have is essentially an end-fed antenna. The coax feed to the un-un may be acting somewhat as a counterpoise also. To see a 1:1 SWR at your antenna analyzer at the shack end of the coax you would have to have a 450 Ohm resistive load at the un-un. Nowhere in the radio spectrum are you going to wind up with a 450 Ohm resistive load at the un-un 450 Ohm port, so you will never have a frequency where the SWR is 1:1. The results you are seeing is about what I would expect.
I was attempting an end-fed long wire antenna as described [here](
https://www.vk6ysf.com/unun_9-1.htm). Their tests were all done with a 450ohm load on the UnUn. Are you saying that load is impossible with the antenna length I chose or generally it is impossible on HF with a long wire antenna?
The antenna will have resonances at frequencies where the long-wire is ¼ wave and odd multiples and where the wire is ½ wave and odd multiples. Where the resonant frequency of 1/4 wave multiple is the un-un 9:1 port end impedance is probably in the neighborhood of 45 to 70 Ohms, a 6.5 – 10:1 SWR at best and for ½ wave resonances the wire end is around 4000 Ohms – a SWR of 9:1 at best. Between the resonant frequencies the antenna feed point is very reactive and so the impedance is high and the SWR will be very high – much greater than 9:1. The coax will interact with the antenna impedance also to alter the SWR reading and resonant frequencies at the shack end of the coax. Your un-un may be o.k. I presume the output side of the un-un has one terminal tied to the coax 50 Ohm input shield point.
Just guess-timating the lowest frequency dip points that you may be seeing are around – 3.1, 4.0, 6.2, and 9.3 MHz. Just curious, is this close to what you see? There will be lots of additional resonances above 9.3 MHz.
The dips are really tiny so its really hard to read on my analyzer (RigExpert AA-54) but the lowest 'dip' (or should i say blips?) appears be around 7.8mhz and 11.8mhz. There is also a very slight decline in SWR starting around 7.5mhz as you work your way up to 30mhz. This decline brings you from maybe 10.6:1 down to 10.2:1.
I’m curious what your receiver is and what frequencies you are interested in listening to, there is now ham cw transmissions below 500 kHz and do you want to primarily listen to 20 – 10 meters.
I have an old National NC-140 I posted about in another thread a few months ago (still haven't replaced the capacitors and tuned it up) and an Eton Elite 750. Once I upgrade my license I plan to buy a modern transceiver to get my chops up with before I dive into restoring any boat anchor transmitters (which is my ultimate goal).
I expect to build a new antenna before I get into any HF transmitting and really only hope to use this antenna for listening to the higher HF bands and 'world band' radio stations.
I wonder where the power-line feed is in your neighborhood. There is so much powerline noise and digital RFI today from all of the computers, lights, wall-warts, etc. The antenna should be as far from the house and powerlines as possible.
I'm glad you brought this up. My house is on a 1/2 acre property with a high voltage power line running behind the property. The house is at the front of the property which puts it roughly 345 feet from the high voltage line. I read somewhere that running your antenna perpendicular to the power line can help with interference, so I did that. My power comes in from the front of the property and my transmission line enters the house a few feet from my power meter.
The further I move the antenna from the house, the closer I move it to the high voltage line.
I tend to think you will be better off with a balanced dipole (center-fed) out over the back yard as high as possible, fed directly with the coax or a little better with a 1:1 current balun at the center feed point which helps to isolate the antenna wires from the coax.
I would love to build a dipole tuned to 80 or 160m eventually, but my goal at the moment is to have a broadband antenna I can use for more general SWL. Obviously I haven't achieved that with this antenna, but is there some other broadband designs that would be feasible for a beginner to assemble?