Homebrew Off-Center Fed Dipole Horizontal vs Vertical?

Status
Not open for further replies.

RedPenguin

Member
Joined
Feb 28, 2007
Messages
1,081
Many scanners use a 10.7MHz IF, meaning they have a local oscillator (internal transmitter) that is tuned 10.7Mhz away from the desired frequency to be received and the mixer in the scanner converts the desired frequency to 10.7MHz for further filtering and processing downstream. Its common for many scanners to radiate its internal local oscillator out through the antenna jack and into the air for a short distance.

Let's see, your 464.175 frequency in the Regency, minus 10.7MHz for its internal local oscillator equals 453.475MHz, what a coincidence!

Try disconnecting any external antenna on the Regency and see if the interference goes away. If so you can probably use a very low gain, low noise amplifier in front of the Regency and that will reduce the radiated local oscillator interference quite a bit. There is a good chance its just leaking through a plastic case on the Regency and aside from shielding the case you might not be able to improve things much.
prcguy

Looks like I am probably just out of luck.

It seems the Regency can broadcast the RFI in to different rooms even if you remove the antenna.

EDIT: To "solve" the problem, we just set the Regency to 469.175 (Input) as they really only cared about local traffic not traffic from other areas that comes across the repeater.

So they still get to hear exactly what they want without causing problems on the other channel.
 
Last edited:

nanZor

Active Member
Joined
May 28, 2009
Messages
2,807
Probably because its radiation pattern is lousy at the horizon on UHF with lobes pointing up and down and if you turn it on its side you can point a gain lobe at what you want to hear. Try turning it in horizontal mode and see if you get a peak and a null on the signal.
prcguy

Heh, that's what Cheek and I were thinking when usenet was big. :) The original intent was never to make it vertical due to the funky lobes, but was a fun one-off to just throw in the corner at any angle for local non critical stuff.
 

nanZor

Active Member
Joined
May 28, 2009
Messages
2,807
Neither amp seemed to work well without the FM Trap before when I tried using them a while back.

Yep - FM overload is a big issue. You'll notice that the antenna is really a dipole resonant on the FM broadcast band. You can place the feedpoint of a dipole anywhere along it's length, as long as you match the impedance properly.

Back in the day, the idea was "what about an 88mhz dipole operating on harmonics, the 10th being near 880 mhz or so?" Let's multiband it with a lot of impedance peaks and valleys by offsetting the feedpoint too.

Purposely pointing a narrow lobe at stations was fun when used mounted horizontally, but to do it right you need a very stable mount / rotator etc. At that point it was better to just use a yagi.

What was to be a weekend project goof that you threw into the corner turned out to be popular with non-critical local systems used vertically.

Aside from the funky lobes of what is really a longwire above 150mhz, FM broadcast overload was common necessitating a trap...
 
Status
Not open for further replies.
Top