For a receiving antenna, you can run a simple to test to see if you can
do better with respect to antenna length. Find a frequency where there
aren't any signals. Disconnect the antenna. If the noise level
decreases, then you're doing as well as you can. Any change in the
antenna length that increases the signal level will also increase the
noise level, which won't make you hear any better. It's possible that
the test will go one way at one frequency and the other way at another.
If that happens, you'll have to experiment to find the best compromise,
or arrange to have different lengths for different frequency ranges.
There's no way to predict what length is optimum, because if you're
using a single wire antenna, the actual conductor doing the receiving is
not only the wire but also any path from your receiver to the earth.
This typically includes some amount of house wiring.

This isn't to say that you can't make a better receiving antenna at all.
Putting a balanced antenna high and in the clear, with steps taken to
eliminate feedline pickup, will reduce any noise being picked up from
the house wiring and other nearby sources. And making a directional
antenna will increase signals more than noise -- although only in some
directions, at the expense of others. But in the meantime, while you're
staying with a random wire, the simple test will tell you what you want
to know.

Roy Lewallen, W7EL

wrote:
Hello,

I have just gotten my radio license and someone gave me an old
hammarlund receiver that hadn't been used in over 25 years. I used a
variac to bring up the power slowly, because I was told that the
capacitors needed to be reconditioned. Everything seemed to work fine
and I was able to pull in some transmissions with a random wire
antenna. I don't have a transmitter yet, so I am hoping to just use a
longer random wire antenna, laid along the attic until I can get money
together etc. My question is probably dumb, but I was wondering if
there is an "optimal length" for a random wire antenna for hf?

THanks for any help you can provide,

Joel Hainley
www.hamtesting.com