Long Wire RX Antenna - How Long is Too Long?

[ultravista]

Is there a point of diminishing return when it comes to the length of a long wire RX antenna?

In other words, does doubling the length provide measurable benefits?

I ran a 500 foot spool of 14 gauge wire through the woods this weekend. Run through screw-on insulators at a height of 14 foot. 500 foot is a long run, I underestimated just how long it would be.

So ... will this 500 foot long wire 'capture' more signal say a 250 or 100 footer?

[dxace1]

The subject of "Beverage" antennas -- super long wires has been written about frequently
in the SW press over the years.   There are many articles on this.  If you do a search online
you will bring up many videos and articles.

[ChrisSmolinski]

Even a few posts here on the HFU about Beverage antennas. I have two of them, they work very well.

[ultravista]

I ran tests last night and this morning and have to say, I am surprised that the 500 foot wire and Pixel 1B were nearly identical. The antennas are connected to a Palomar 4 port switch box.

Yaesu FT-950 > RSP2 (panadapter) > HDSDR with auto-gain disabled to keep settings constant between both antennas.

Tuning from LW through HF, both were comparable in signal strength (audibly/visually in HDSR). The Pixel at some frequencies was a slightly better or worse than the wire. This I attribute to the position of the loop being static and nulling some/improving some.

At LW, I can routinely pickup a NDB @ 415 khz. It was slightly better with the Pixel.

I thought the 500 foot wire would outperform the Pixel. It did not ... and I would like to know why.

How can a 4 foot loop, 4 feet off the ground, perform as well as 500 feet of copper 14 feet off the ground?

[ChrisSmolinski]

First a question, how is your antenna fed? That is, the connection from the radio to the antenna itself? Please be as precise as possible.

Some random notes:

Beverage antennas are directional towards the far end of the antenna, provided the antenna length is terminated and large relative to the wavelength (at least one wavelength, ideally longer) At 415 kHz, this is not the case for a 500 ft Beverage so I would not expect it to be directional, at least in that way. It probably has a nice assortment of lobes and nulls however.

Unterminated, the Beverage will also pick up in the opposite direction (but see above for the length/wavelength relationship)

Normally a Beverage is run about 6 ft above the ground. At 14 ft, that is going to change things.

Beverages are considered high directivity / Signal to Noise ratio antennas. You're not looking for just more signal, which is not meaningful. You're looking for a better signal to noise ratio, so I'd compare that way.

Check up on the higher bands, say 4 to 10 MHz, and see how the two antennas compare, especially with stations located in the beam of the Beverage/not really a Beverage antenna.

[NJQA]

One other item to consider is your antenna switch and coax.  If the isolation is not so good, you can have significant signal leakage between the antennas you are running your A-B tests on.  This can make the poorer antenna seem to perform better than it actually does.

[RobRich]

You have a low "longwire" antenna. Like Chris noted, it is directional off the ends if end-fed, especially as frequency increases to multiple antenna wavelengths.

Given the close proximity to the ground, and end-fed longwire is going to act like an unterminated beverage as antenna length increases above like 1.5-2x frequency wavelength. A beverage is a negative-gain travelling-wave antenna. Beverage "gain" drops even further into negative numbers as antenna height lowers to the ground due to increasing losses. The idea of a low longwire antenna like a beverage is to improve directivity and signal-to-noise ratio; not explicitly to increase gain.

While a different type of antenna, your small loop is also a negative gain antenna designed for directivity and SNR. However, it also benefits from a ~30dB preamp. Put a decent 30dB preamp on your longwire, preferably at the feedpoint like the loop does, then you can get a better idea of comparative gain between two antennas. ;)

Anyway, remember signal-to-noise ratio is the important thing here. Orient your loop to have the same directionality of your wire. Compare their noise floors, then compare their signal levels to determine their actual SNR difference. You might be surprised.

A primary case in point is an antenna like my 148' loop-on-ground antenna. I often listen to HF transmissions under S1 signal strength when using the LoG without a preamp engaged, yet the SNR is plenty enough to have an easy copy. I care more about hearing and copying transmission than what my signal meter reads. Likewise SNR is why SINPO or SIO is typically more useful for reports than just S levels alone.