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Mini auto AM/FM antenna for HF receiving?

Discussion in 'Mobile Radio Systems' started by K5DH, Oct 26, 2018.

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  1. K5DH

    K5DH Ham Member QRZ Page

    I see those little stubby AM/FM radio antennas on many new cars these days, including two of my own, and I assume they're some kind of voltage probe design or similar. Since they receive across such a wide range, I keep wondering if one could be used as an HF receiving antenna, not necessarily in a car, but perhaps in a home setting. This is just me hatching silly ideas. . .

    Anyone tried this?
  2. K0BG

    K0BG Platinum Subscriber Platinum Subscriber QRZ Page

    Size matters! Enough said!
    KE5PPH, W8ARD and KU4X like this.
  3. KD4UPL

    KD4UPL Ham Member QRZ Page

    I have a car with one of those. It doesn't pick up FM worth a darn and AM is even worse. Don't waste your time on tiny antennas.
  4. WD5IKX

    WD5IKX Ham Member QRZ Page

    For HF..."Go BIG or go home" (^8

    John WD5IKX
  5. AG6QR

    AG6QR Subscriber QRZ Page

    Well, anything can work. A few years ago, a ham friend of mine was complaining about a noise on 40m, which I wasn't hearing at my home, a mile or two away. To track it down, I decided to take my KX3 mobile. I had no good HF antenna, but I did have a mag mount antenna designed for 2m.

    I wasn't expecting much, and for that purpose, low sensitivity was perhaps an advantage. But I was surprised that I could pick up international broadcasters and many ham stations from around the US.

    One should never use a small mismatched antenna for transmitting. The SWR will be terrible, the transmitter could be damaged, and the signal won't be radiated well. But for receiving, a small inefficient antenna can be ok. Many portable shortwave receivers come with a short whip. For receiving, it's all about SNR, and an inefficient antenna attenuates both signal and noise equally.

    I'm not suggesting it will work as well as a proper antenna, but for recieving, it may be good enough, and worth a try.
  6. G3YRO

    G3YRO Ham Member QRZ Page

    Yes, they rely on the fact that the FM and AM broadcast signals are VERY strong !

    Try fitting a bigger antenna on your car radio, and see how much better it is on both AM and FM. I have a 2m 5/8 wave whip on one of my cars, which is used by the car radio when I'm not transmitting, and it's great on the broadcast bands.

    Having said that, I operate 160m mobile from my other car (a little Fiesta), and swapped the original tiny antenna on the roof for a 3/8" mount, so I could fit my base-loaded 160m whip.

    However, I don't leave it on all the time . . . instead I fit a small 19" whip, as the car radio still uses the same antenna feed (via a relay, when the Transceiver is turned off)

    But sometimes I listen on 160m on that tiny un-loaded whip, and can actually hear strong signals at the other end of the country!

    Roger G3YRO
  7. KB0MNM

    KB0MNM Ham Member QRZ Page

    Some of those 'shorties' rely on an amplifier to pick-up and listen stronger only on the broadcast AM and FM frequencies, although strong signals from other bands can get through. Theoretically, the best HF reception might be the same as an 'old-fashioned-transistor-radio' dial- for both A.M. in the Khz. range and FM in the Mhz. range. This would explain why 160 meters ( very low frequency ) might be easier to hear than 10 meters ( think 11 meters, that would be around 29 Mhz. ). Since the ham bands are 1.8 to 30 Mhz., the broadcast FM portion of that 'stubby' antenna amp ( if it has one ) would not be doing amateur HF reception much good. If you were to try to transmit on an antenna system that had a built in receive-only amp system- you would probably incur a bill for $200.00 or more at the auto dealer, or $50.00 plus untold miles finding the 'amp' in a junkyard ( if you want regular AM/FM broadcast restored ). It is your vehicle... disconnecting the antenna lead might or might not be enough- because the 'amp' may be at the tail end of the vehicle. Some are powered by the radio over the antenna lead, others by a separate DC feed to an area often associated with the underside of a 'deck' third tail-light in the trunk compartment. Getting schematics for these might be close to impossible, unless you just want a 'reverse-engineered' drawing or have advanced knowledge of FCC equipment authorizations.
  8. G3YRO

    G3YRO Ham Member QRZ Page

    I personally haven't come across any cars with built-in car antenna pre-amps . . .

    Not since the era when they tried using the Heated Rear Window element as an antenna . . . these had a unit with RF chokes to isolate the DC, plus a pre-amp to feed the radio. They were still pretty poor - the main problem was that they were directional!

    The main thing to bear in mind is that the coax used to feed Car Radio Antennas is special . . . it's very low capacitance/high impedance. (otherwise you'd get almost nothing out of the short whip on the AM Band). And even so, the front end of the radio compensates for this on AM . . . the best radios have a trimmer, accessed through the fromt panel, to peak the antenna fitted to your particular vehicle.

    If ever you replace the broadcast antenna with a proper mount, to use an HF or VHF Mobile Antenna (like I have on both my cars), you need to run PROPER coax to it.

    So I can still use this antenna on the car radio, I have fitted a relay at the base that connects the original co-ax line through a small relay, so that the car radio still gets a signal. This switches automatically when you turn the car radio on, using the DC Antenna Feed that most radios have (designed to operate an electric telescopic antenna).

    I use that same DC feed to operate another relay that automatically switches the driver's door 'speaker between the car radio and the Rig in the car !

    Roger G3YRO

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