Half-Rhombic antenna termination value?

[JasonW]

Hello All,

This query is not strictly on-topic, but because the information I'm seeking would also be useful to FM and Short Wave listeners (especially DXers), I figured I'd go ahead and post it here. I took down my modest 1/4 wavelength 11 meter (CB) whip antenna atop a 20' mast after a few too-close-for-comfort lightning strikes this summer.

To replace it, I'm looking into putting up four switch-selectable, terminated half-rhombic antennas mounted on or just above the ground, against the walls of my house. (The half-rhombic looks like an inverted-vee and is vertically polarized, but it is highly directional just like a regular "full" rhombic.) At CB wavelengths, making a practical-sized half-rhombic with sides 1 wavelength long is not a problem.

However, there seems to be no standard value for the termination resistance of a half-rhombic antenna. This web page ( http://www.anarc.org/naswa/issues/199911/lib199911.html ) says that a half-rhombic mounted above a ground plane should have a terminating load resistance of 300 ohms, while these web pages ( http://www.pacificsites.com/~brooke/OE303.shtml , http://www.flmars.org/halfrhombic.htm , http://www.sniper-training-guide.com/training-guide/000283.php , and http://www.deltagearinc.com/Communication.htm ) call for termination values anywhere from 400 ohms to 700 ohms. I'd like to go with 300 ohms because I have a few 6:1 (50 ohm:300 ohm) QRP HF voltage baluns, but I don't want to smoke my transceiver. Has anyone here ever used a half-rhombic antenna, and if so, what termination value worked best for you?

Many thanks in advance to anyone who can help. -- J. Jason Wentworth

 

[BlitzK]

> Hello All,
>
> This query is not strictly on-topic, but because the
> information I'm seeking would also be useful to FM and Short
> Wave listeners (especially DXers), I figured I'd go ahead
> and post it here. I took down my modest 1/4 wavelength 11
> meter (CB) whip antenna atop a 20' mast after a few
> too-close-for-comfort lightning strikes this summer.
>
> To replace it, I'm looking into putting up four
> switch-selectable, terminated half-rhombic antennas mounted
> on or just above the ground, against the walls of my house.
> (The half-rhombic looks like an inverted-vee and is
> vertically polarized, but it is highly directional just like
> a regular "full" rhombic.) At CB wavelengths, making a
> practical-sized half-rhombic with sides 1 wavelength long is
> not a problem.
>
> However, there seems to be no standard value for the
> termination resistance of a half-rhombic antenna. This web
> page (
> http://www.anarc.org/naswa/issues/199911/lib199911.html )
> says that a half-rhombic mounted above a ground plane should
> have a terminating load resistance of 300 ohms, while these
> web pages ( http://www.pacificsites.com/~brooke/OE303.shtml
> , http://www.flmars.org/halfrhombic.htm ,
ht> tp://www.sniper-training-guide.com/training-guide/000283.php
> , and http://www.deltagearinc.com/Communication.htm ) call
> for termination values anywhere from 400 ohms to 700 ohms.
> I'd like to go with 300 ohms because I have a few 6:1 (50
> ohm:300 ohm) QRP HF voltage baluns, but I don't want to
> smoke my transceiver. Has anyone here ever used a
> half-rhombic antenna, and if so, what termination value
> worked best for you?
>
> Many thanks in advance to anyone who can help. -- J. Jason
> Wentworth
>


I wouldn't terminate it. I'd run it through a good antenna tuner.

 

[JasonW]

I'll give it a try. From what I've read, un-terminated rhombics and half-rhombics aren't closed at the end opposite the feedpoint (which would form a closed loop antenna with its attendant problem of high voltage arcing across the antenna tuner's capacitor plates). -- Jason

> > Hello All,
> >
> > This query is not strictly on-topic, but because the
> > information I'm seeking would also be useful to FM and
> Short
> > Wave listeners (especially DXers), I figured I'd go ahead
> > and post it here. I took down my modest 1/4 wavelength 11
>
> > meter (CB) whip antenna atop a 20' mast after a few
> > too-close-for-comfort lightning strikes this summer.
> >
> > To replace it, I'm looking into putting up four
> > switch-selectable, terminated half-rhombic antennas
> mounted
> > on or just above the ground, against the walls of my
> house.
> > (The half-rhombic looks like an inverted-vee and is
> > vertically polarized, but it is highly directional just
> like
> > a regular "full" rhombic.) At CB wavelengths, making a
> > practical-sized half-rhombic with sides 1 wavelength long
> is
> > not a problem.
> >
> > However, there seems to be no standard value for the
> > termination resistance of a half-rhombic antenna. This
> web
> > page (
> > http://www.anarc.org/naswa/issues/199911/lib199911.html )
> > says that a half-rhombic mounted above a ground plane
> should
> > have a terminating load resistance of 300 ohms, while
> these
> > web pages (
> http://www.pacificsites.com/~brooke/OE303.shtml
> > , http://www.flmars.org/halfrhombic.htm ,
> ht>
> tp://www.sniper-training-guide.com/training-guide/000283.php
>
> > , and http://www.deltagearinc.com/Communication.htm ) call
>
> > for termination values anywhere from 400 ohms to 700 ohms.
>
> > I'd like to go with 300 ohms because I have a few 6:1 (50
> > ohm:300 ohm) QRP HF voltage baluns, but I don't want to
> > smoke my transceiver. Has anyone here ever used a
> > half-rhombic antenna, and if so, what termination value
> > worked best for you?
> >
> > Many thanks in advance to anyone who can help. -- J.
> Jason
> > Wentworth
> >
>
>
> I wouldn't terminate it. I'd run it through a good antenna
> tuner.
>