Larsen NMO 2/70 antenna deaf on HF

[rpjjpr]

I just installed a Larsen NMO 2/70 antenna on my car. While I wait to get a VHF/UHF installed, I figured I would mess around with the Yaesu FT1XD. The FT1XD has wideband coverage down into HF. I can't hear anything. I was checking the connections and found that if I take the antenna off and remove the whip and touch that to the center conductor of the NMO mount in the roof of the car I can hear HF. There must be something in the base of the antenna that is killing HF. I have used listened to HF with a small whip and c copper VHF J-pole before. So, I'm just kinda stumped. I was just wondering if someone had a simple answer or could point me in the right direction to learn more about what is going on. I was excited to catch some AM pirates while on the go.

[Josh]

Some v/uhf ants are at dc (but not v/uhf rf) ground so hf isn't going to be stellar, might consider a length of wire to plug in and toss out the window when you wanna hf.

[Stretchyman]

JOSH is correct.

There will be an inductor in parallel with the antenna output and whilst hiZ at V/UHF will be a DC short at lower frequencies.

Str.

[rpjjpr]

Thanks for the info and direction.

[RobRich]

All you should need for basic shortwave reception in your deployment scenario is a short simple unloaded whip and perhaps a NMO to 3/8"x24 adapter if required.

Even just like a 3' to 6' whip should suffice, as much longer likely will overload your wideband handheld, which is a common issue with such handhelds at HF and lower frequencies.

[Elf36]

I agree with everything said here, but if you have an issue with the antenna on VHF/UHF, you might want to remove the base of the NMO mount and see if the metal tab that touches the base appears to make contact. As a general rule I have always taken off the base and pulled down on the tab, so that I know its going to have pressure on it when it's put back together.

[ThaDood]

You may have an existing 30" AM / FM RX'ing antenna that you can already use on your vehicle. They certainly work better than those new short stubbies that come on vehicles today. That 10", 45deg, AM / FM stub antenna on my sisters 2012 Chevy Cruz just sucks.

[Ray Lalleu]

A general conclusion about VHF antennas to be used as HF receiving antennas with hand-helds or small SW receivers :

- that can be tried, but only if there is no DC short between the core and the shield at the low end of the coaxial line
- HH and small SW receiver are very sensitive and can work well with a very small antenna, provided the antenna input at HF is at high impedance (not of the 50 ohms family, but don't be fooled by the socket type)
- if there is a DC short seen at the low end of a former FM or TV receiving antenna you are no longer using, then you have to go in the attic or above the roof where the old antenna is placed and find a way to remove the short (generally, there is a folded dipole or a delta or gamma-match, so the best would be to leave the old antenna connected only to the screen of the coax, and to place a whip or a short length of vertical wire above that old antenna, with that new 'whip' connected to the core of the coax downline).
- before going in the attic or on the roof, try with your HH/SW receiver is working well on HF with just a 3ft/1m long wire inserted in the center of the antenna input socket !

[Josh]

I recall reading a Navy hf rx antenna study where they determined a 6ft vertical was about all you needed in most cases, but you have to consider the groundplane under that 6ft vertical being tested was a warship - typically a few hundred feet long and maybe 60 feet wide. A very common vertical shopboard antenna is just about flagpole height/length, and you will find these on just about any US warship.

I once had a cb 1/4w up about a wavelength on steel pipe masts and found it performed relatively spectacularly on hf, and even down into lf and vlf after a ground wire was added - the piping didn't make for a very good ground in itself. Doesn't take much for hf rx as most rx have excess gain, also the vertical excels at dx as it responds to lower angles of radiation than a typical horizontal dipole. Just some musings for ya.

[RobRich]

Pretty much the reason I have said if a person has already a decent VHF/UHF discone to replace (if possible) the usual 6m loaded whip with a regular ~9'  whip to add HF reception. It is not a weak signal solution, but it usually works fine for casual HF listening.

[Josh]

The bottom line is if the background noise increases when you plug in an antenna system, it'll do.