Screwdriver antenna,miracle or what?

[W2PFY]

I remember having a conversation with a trucker who just purchased a new tractor and he said it was all electronic including electronic fuel injection. He said he was going down the road and decided to fire up his new transceiver, every idiot light on the rig came on and sidelined him to the side of the road! When he installed it, he must have tried it without the engine running? The same thing would happen to me on my 1988 chevy pickup truck. Every time I would go on 40 meters on AM. On mod peaks the engine would buck. Just about the same time I got a MFJ antenna analyzer and sure enough, the thing could not find a resonant point anywhere on 40 meters. Turns out that the coax shield was open.     

[n1zpy]

Mounting the antenna is the easy part.  Now to get the bonding finished up and the amp wired etc.

[IN3IEX]

Professionals have found this compact solution:
https://cdn.rohde-schwarz.com/pws/dl_downloads/dl_common_library/dl_brochures_and_datasheets/pdf_1/HX002H2_cat_2015_36-37.pdf
Dipoles the size you can use and a suitable tuner. Typical gain across the HF is shown in a plot.
The symmetric dipole avoids ground losses, so that most of the losses are in the tuner, that you can easily upgrade from time to time. The "butterfly" four whips design is useful to increase the bandwidth at low frequencies. Copy this design.

G.

[AB2EZ]

Giorgio

I respectfully disagree with your comment:

"The symmetric dipole avoids ground losses, so that most of the losses are in the tuner... "

An electrically short antenna (e.g. a dipole whose total length is much less than a half wavelength) will have a larger ratio of near field RF signal strength to far field RF signal strength. In the case of a dipole, the smaller the ratio of the electrical length to a half wavelength... the larger the ratio of near field RF signal strength to far field RF signal strength.

The far field electromagnetic modes radiate away from the antenna... and fall off in RF signal strength as 1/r... where r is the distance from the antenna. Most ham radio operators are familiar with that, even if they don't have any background in electromagnetic field theory.

The near field electromagnetic modes do not radiate away from the antenna. The near field RF signal strength falls off exponentially [i.e. as exp(-kr), where k is a complex number whose real part is positive].

Even if the losses in the tuner were zero, and even if the resistive losses in the feed line and the conductors of the dipole were zero... the near field will produce resistive (heat) losses in the ground, and in any nearby conductors that have resistance.

The shorter the dipole is compared to a half wavelength, the larger to ratio of: [the total transmitter power that is converted to heat due to near near field losses] to [the total transmitter power that is radiated away from the antenna in the far field].

Separately, the above situation is similar for magnetic loop antennas that are electrically small... EXCEPT... the near field of a magnetic loop antenna is largely magnetic (rather than electric)... and objects near the antenna may have relatively small magnetic field losses vs. electrical field losses. This is why an electrically small magnetic loop antenna may radiate a larger portion of the total transmitter power... when compared to an electrically small dipole or whip antenna.

Stu

[Steve - K4HX]

Good points Stu. Seems the question becomes: Which has the greater ground loss, a small dipole or a small vertical?

[WD5JKO]

I wonder about this 80-40m magnetic loop adapted to be used on an automobile?


http://www.btv.cz/en/upcoming-news

Jim
Wd5JKO

[n1zpy]

 One of the guys actually attached 60' fine wires to his car bumper and drove down the highway... :-)



Tom, K1JJ

Would mounting a 80m hamstick horizontally at the base of my screwdriver be of any benefit? Also been thinking about 1/4 wave stubs for each band teed into the coax that would be open with a switch at the end of each stub to short it for the band your working.  If I do implement the stubs they would be after the amp.  Amp is not installed yet.  Amp is rated as 1kw input so I'm only expecting 5-600 watts output.

[KD6VXI]

The amount of metal under your car is greater than the amount of metal in a ham stick.  That wouldn't do anything.

As to stubs, your already using a hi Q antenna, so harmonics aren't really an issue.  Stubs do nothing for close in products.

I did use a stub on my mobile, but only for matching.  Then I bought a screwdriver and threw the Monoband antennas out.

--Shane
KD6VXI