3
$\begingroup$

I found this old log periodic antenna that I assume was from a TV. Could I, or how would I, use this on the amateur radio bands? My main focus would be VHF, with UHF being secondary and HF would also be nice. I'd like to keep it as cheap as possible.

enter image description here

Largest: 4' 5" (134.62 cm) - seems to be electrically connected to form a 8' 10" element?

smallest: 5" (12.7 cm) (unfolds to be parallel with another 5" element)

"Other" smallest: 6" (15.24 cm) (by itself)

Improve this question
$\endgroup$

3 Answers 3

Reset to default
6
$\begingroup$

I'm sorry to say that's not a simple log-periodic antenna, it's a hybrid monstrosity to meet some particular need.

Guessing from the element lengths:

  • 136 cm each side would work at about 50 MHz, so low-band TV. Two-element log-P with a reflector. OK.
  • 12 cm each side would work at about 550 MHz, so mid UHF TV. Some sort of Yagi
  • 15 cm total would be a director for about 900 MHz, so the top end of UHF TV.

It doesn't seem to cover the VHF ham band, and it probably only starts working above the 440 MHz ham band.

You could probably re-use the mounting and feed hardware, cut the feed down to work at 145 MHz, and then install new directors to give it some gain. But you are basically building a new yagi, and with very non-ideal director spacing.

If you need a directional antenna (for satellites?) rather build a new yagi from scratch. You can make a nice one in a few hours with some bits of brazing rod and a broomstick.

Improve this answer
$\endgroup$
2
  • 1
    $\begingroup$ I'm wondering, why wouldn't it work on VHF at all? One of the elements (the third largest, see image) is roughly 24 inches, what if I just cut it down to 19.2 for VHF? $\endgroup$
    – Galaxy
    Commented Nov 21, 2020 at 20:08
  • $\begingroup$ Maybe I didn't say this clearly - there's no evidence of the middle-length elements needed for the 2 m band. TV at the time this was built, must have been only a mix of legacy 48 MHz stations, and newer UHF stations. The VHF high band, about 136-300, is too valuable for two way radio so had less or no TV on it. So the antenna is really two in one, a low VHF yagi on top of a UHF yagi, with no useful performance in the gap. $\endgroup$
    – tomnexus
    Commented Nov 21, 2020 at 21:38
5
$\begingroup$

Those antennas are held together with aluminum pop rivets which are easily drilled out. In less than an hour you can completely disassemble a TV antenna like that and salvage the main mast, all the dipole elements, and the plastic element mounts- and use these and some fresh pop rivets and an electric drill to build yourself a two meter yagi beam with excellent performance, for which there are numerous plans available on the web.

Improve this answer
$\endgroup$
2
$\begingroup$

Log-periodic antennas have a very wide bandwidth. One designed for TV would not work at HF, but it may very well work on 2 meters, 6 meters, and some UHF amateur bands.

I suggest that you measure the SWR and see. However, unless its impedance is 50 to 75 ohms, you will need a suitable matching device.

Can you post photos of it? While you're at it, include the minimum and maximum lengths of the elements.

Countless hams have used TV log-periodic antennas on the ham bands, and with little or no modifications.

Improve this answer
$\endgroup$
2
  • 2
    $\begingroup$ Oops, I forgot to add the image. I'll also add the dimensions. I assume the length of the elements would indicate the frequency range? $\endgroup$
    – Galaxy
    Commented Nov 21, 2020 at 4:07
  • 1
    $\begingroup$ @Galaxy Exactly. And if they are at an angle, just measure from the tip of one element to the center of the boom. $\endgroup$
    – Mike Waters
    Commented Nov 21, 2020 at 4:10

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .