Horrible SWR with random wire antenna

[KJ6RSG]

I'm slowly trying to get back into the hobby. At this point I'm just trying to do SWL until I upgrade my license.

I setup a random wire horizontal antenna on my roof last weekend. Its suspended a few feet above my roof and extends out to my laundry line. The full length of the antenna is about 74 feet. I build a 9:1 Voltage UnUn and my transmission line is about 40 feet of RG-58 going into my living room.

I tested the SWR across the HF spectrum and it was solidly above 10:1. There was an extremely minor dip around 28MHz but it was still above 10:1.

This weekend I am going to take down the UnUn to check the SWR without it and to test the UnUn on my bench with a resistor load. Is there anything else can I do to try to diagnose this antenna?

[W7TFO]

You might try a counterpoise (shield to that) just on the rooftop without a balun.

73DG

[Tom WA3KLR]

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.

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.

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 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 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.

[KJ6RSG]

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?

[KJ6RSG]

You might try a counterpoise (shield to that) just on the rooftop without a balun.

73DG

I'll try it without the balun this weekend. Could the counterpoise just be a additional wire on the roof and attached to ground?

[W7TFO]

Essentially, yes.

73DG

[KB2WIG]

FWIW,  you don't need much of an antenna for SWLing. For now, I would put up a 10m dipole, fed with coax, and use it for listening. A few reasons....

On HF, usually you have enough signal for most receivers to copy on the bands.
You could get on 10m if you have a xmitter.
You can move it around a bit easier to find a lower noise level.
You can always put up more than one ant.... probably .
Screwing around with an end fed is such a pain.....

Gud luck,

KLC

[WA4WAX]

First: Make a GOOD 1:1 current balun. See K9YC choke cook book.  You will need 2 2.4 inch #31 cores, and an outdoor plastic NEMA box.  Plastic, not metal.  Also, get about 30 feet of #12 solid copper wire, with 2 different insulation colors for each wire of the winding.

I would use 11 turns on each core, with the two in series.  That will get you lots of CMR on 40 through 10 meters.

Second: Cut wire to resonance at about 7125 KHz.  This is a reasonable choice.

Third: Make some 600 ohm OWL, about 45 feet long.  More on this later.  The bottom 18 feet or so should be bare.  In fact, the whole thing can be bare copper.  #14 good.

.....To be continued.

[WA4WAX]

OK......here is what to do.

Connect the OWL Zepp feeder to the end of the antenna.  One wire will be connected, the other free.  You might connect a 3/8 or 1/2 inch copper pipe cap to the free end for corona mitigation, especially if you will transmit at some point.

For 40 meters, short the Zepp feeder at about 35 feet.  Bond the balun output leads about 30 inches above the short, and check the SWR at balun input. Adjust the short bar until Z is pure resistive.  If R greater that 50, lower the tap point, and adjust short for pure R again.  You will eventually hit 50 or something close.

If R less that 50, move tap point up.  Go through the same iteration.

Run coax from balun to receiver.

On 20 meters, short the Zepp at about 45 feet.  Repeat procedure above.

You may do the same on 15 and 10 meters.  As you can see, the Zepp is shorted at an odd multiple on 1/4 wave.

Have fun.

[WA4WAX]

More:

If you can get a balanced L tuner, put a 600 ohm noninductive resistor across the terminals.  Two 300 in series best, as you can verify balance with a scope or vtvm.

Procedure: Adjust tuner to 50 ohms at the input.  Remove resistors, and replace with a run of 600 ohm OWL out to the Zepp feeder.  Adjust tap point and short until you get 50 ohms back at the tuner.  When you do, you will have 600 ohms at the tap point.

Last point: Lose the poverty coax.  Get some Mil Spec RG-213.......or good RG-8X at the very least.

[WA2AAE]

To get good operation of an end fed antenna, using a 9:1 unun, the length is critical. There are many web pages that discuss what lengths of wire to use for this type of antenna and what lengths to avoid.

My end fed antenna is 53 feet long (one of the good lengths recommended) with a 9:1 unun and 25 feet of coax feed line. On 40 and 20 meters my SWR is less than 2:1 across the bands. Also make sure your unun is really 9:1 using a resistive load to check the windings. I've wound a few that I thought were right until I put my antenna analyser on it.

Harvey, WA2AAE

[WU2D]

You can not beat a 49M (or 40M)-31M-25M Fan Dipole fed with RG-6 75 Ohm TV line or RG-58 for cheap and effective SWLing. Throw a 1:1 balun or a few beads up at the feed.