Ham Radio Forum

I have become interested in shortwave radio monitoring after about a 15 year break from the hobby. I'm attempting to build a large inverted-V halfwave fan array in my attic and have been scouring the 'net for days looking for the best way to proceed.

I still have a question about one point in particular. Some websites claim that in addition to the primary frequency an inv-V dipole is cut for, it is also sensitive to frequencies on all multiples of that frequency. For example, if I cut a dipole for 6 MHz, it will also pick up well on 12, 18, 24 MHz, etc...

Other websites claim that that only the third harmonic has special significance for this type antenna. (i.e. a 6 MHz antenna will efficiently pick up 18 MHz, but not particularly well on 12 MHz)

Since I want to minimize the number of wires dangling up in the attic, I'd like to use this multiple harmonic deal to my advantage if possible. Can someone give their opinion on which of these two theories (if either!) is correct?

Thanks!

Tim A.
Wilmington, NC

Tim,

You raise some interesting issues. First, it should be noted that the characteristics of antennas installed in attics tend to be far less predictable than where similar antennas are installed in the clear high above ground. The close proximity to earth alone significantly changes an antenna's characteristics. Even worse, most attics contain electrical wiring and metal plumbing vent pipes that a nearby antenna will couple to such that the wiring and vent pipes will become active and important parts of an attic antenna system. Many attics also contain large metal air conditioning ducts. Some contain metal-lined insulation bats. The attics of large homes sometimes contain metal structural members, such as steel I-beams along ridge lines. Even where structural members are all wood, most roofs have metal flashing and often are rimmed with metal rain gutters and downspouts. Metal attic exhaust fans are common in some areas. There may be metal siding outside the ends. Except possibly for the case of a log cabin in the mountains where there is no electrical power or indoor plumbing, attics tend to be far less than desirable places to install antennas. However, desirable or not, sometimes there is no other reasonable alternative.

The harmonic characteristics of an inverted-V fan dipole installed in the clear high above ground are well known, but it is impossible to predict the characteristics installed in a typical attic without precise information about the height about ground and the exact dimensions and position of every metallic object within a wavelength or more. For the case of an inverted-V fan dipole installed in the clear high above ground, the seemingly conflicting things you have read about the antenna being reasonably useful only at odd harmonics or at both odd and even harmonics are both true, depending on how the antenna is fed.

The feed impedance will be relatively low at the frequency of half-wave resonance and at odd harmonics of that frequency. (The actual impedance will depend on the angle of the V, the angle of the fan, the number of wires used, and to a minor extent, their diameters.) If such an antenna is fed in the center with a low impedance transmissions line, the antenna will be reasonably usable at the fundamental and at odd harmonics. However, the feed impedance will be somewhat different at each harmonic and the antenna radiation pattern will vary considerably at each harmonic.

The center feed impedance will be high at even harmonics and the antenna will be reasonably usable at even harmonics if a high impedance (open wire) feed-line is used. As with the case of a low impedance feed-line, both the radiation pattern and the feed impedance will be different at each harmonic. However, feed-line losses due to impedance mismatches will be far less than for the case of a low-impedance line.

If an antenna tuner is added between the receiver or transmitter and the feed line, and if a high-impedance feed line is used, the antenna can be used at both odd and even harmonic frequencies. In fact, such a combination will work reasonably well even at frequencies in between odd and even harmonic frequencies. That is by far the most flexible arrangement, but it has two disadvantages. (1) The cost of an antenna tuner (not much where an antenna is used only for receiving). (2) It will be necessary to readjust the antenna tuner to achieve a good impedance match at different frequencies.

-Bob

Thanks so much for your reply Bob!

Thankfully my attic doesn't contain ventilation ducts and only has limited wiring for a couple floodlights pointed at the backyard. The other points you bring up about the effects of metal flashing and metal gutters are unfortunately going to be true in my case.

I used the excellent program here on the S-Meter website to refine the exact length needed for the dipole fan segments. By playing around with the input variables, I was able to see the effects you mentioned about proximity to ground and the angle of the V. Since I realistically won't be able to account for every possible variable in my particular case, I feel much better knowing I was able to account for at least some of them. This should help me achieve better reception than I would have by using the "standard" 468/f equation quoted on many websites.

Thanks again for all your help. This website is certainly one of the internet's best kept secrets!

Tim A.
Wilmington, NC