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hi all i have a 80m dipole set for 3.775 Mhz,it seems to work rubbish,ive just done some adjustments,,take a look at the pics of it on the vna,does it look ok,views welcome,cheers m3vuv.ps,forgot to say its 2 80m mobile whips in a dipole config,up at about 12 feet for nvis .

On Tue, 10 Mar 2020 05:54:55 -0700, you wrote:

hi all i have a 80m dipole set for 3.775 Mhz,it seems to work rubbish,ive just done some adjustments,,take a look at the pics of it on the vna,does it look ok,views welcome,cheers m3vuv.ps,forgot to say its 2 80m mobile whips in a dipole config,up at about 12 feet for nvis .

The first 2 pics look OK,
but then you did something wrong to your VNA.
Usually 80 m wire antennas are very narrow
and cannot cover the complete 80m band.
To make it broadband use multiple wire arrangements.


OE8UWW

Using mobile whips it will be rubbish. Its not a dipole - its 2 loading coils

I know its narrow band but just its for a spot frequency of 3.775Mhz,heres some more pics of it and screen shots of it on my foxdelta analyser.also a couple of pics of my verticle.73

On the whip - loading coils....

The (B)and(W)ith of an antenna is determined by it's (Q)uality factor, thus of the R L and C components that make an antenna:

L by the length of the whips

C by he surface of the whips

R defined by diameter and material of the whips

The quality factor of a combination of RLC is given by this formula

Q = 1/R *SQR(L/C)??? and BW=1/Q

Just like a resonance circuit . Note that this formula is more complex if you need real values; just for understanding.

A wire antenna with infinitely? thin wire? would have only L and no C because it has no surface to create capacitance.

the quotient of L/C is very high, and the BW will be very small

To increase bandwidth, add capacitance by using thick wire or even tubes, or add a plate at the end of the wire.

Increasing R will work also, but at the cost of signal losses by dissipation.

Gert Gremmen

--
Independent Expert on CE marking
EMC Consultant
Electrical Safety Consultant

Your two EXTREMELY short whips configured as a horizontal dipole will have very high losses and low efficiency? because they are physically short. Even though the whips load the elements to achieve resonance their efficiency is going to be horrible. This is the price you pay for "convenient" short antennas at low frequencies. Remember SWR has nothing to do with how well an antenna can radiate or receive an RF signal. A 50 ohm resistor will give you a 1:1 SWR but it will be useless as an antenna.

I generated a very quick & dirty model of your antenna and for NVIS signals arriving from straight overhead the difference between a full-sized dipole and your two short whips at 12' is on the order of >20 dB. This is a huge difference so when you say it works like rubbish it is in fact working as it should.

Gedas, W8BYA EN70

Gallery at
Light travels faster than sound....
This is why some people appear bright until you hear them speak.

Another way to describe two 8ft whips as a dipole... A HF rubber duck antenna.
Or an Electrically small antenna (under .1 lambda , for 80m thats 8M long)
and the loading coils or matching scheme will contribute inefficiency.

Its efficiency is typically under 5% maybe worse. Also likely lower HF antennas
less that very high (at least 20M or more) is also a hit to performance as most
of the RF radiate due to ground bounce is straight UP.

Also it has a very small Aperture, so it intercepts a significantly smaller part
of the incoming signal.

NOTE: a full sized dipole at 10ft (3M) height would greatly outperform that
antenna for distances under 300miles (`500km). It can be bent to fit available
space with some reduction in performance.


Allison
-----------------
No direct email, it goes to bit bucket due address harvesting in groups.IO

Looks like an interesting way to get on 80 meters which is not something
everyone has the airspace to do. If you want to play with it try moving
the two whips away from each other with a copper pipe in the middle. Even
four feet will make a difference as that is where this antenna needs to
radiate the most.
NE7LS

Hi nanovnauser;

A couple of more or less encouraging comments for you. Your VNA antenna measurement results indicate the following:

1) You did well... You are using the VNA and interpreting the results correctly in the context that you have described for its use. You clearly have done a fine job matching your antenna. It looks spot on and well matched.

2) Anything that gets you on the air is a good antenna, and I applaud your effort in that regard. 20 dB below 100W puts your EIRP (very isotropic in your case BTW) at the 1W level. Time of day (interpret this as night), prevailing ionospheric conditions, and persistence can render you surprise and enlightenment. WAS has been achieved at considerably lower power levels on 80 meters, and I have personally copied a 10 mW beacon at 340 mil on 80 meters, so I'm not going to rag on your antenna. I rag on those who whine about HOA's and the like for keeping them from operating. Do a Google Earth fly over of N2PK's QTH... He manages to keep the hostile HOA police in the dark. Then take a look at his DXCC record on 160 meters, and his position on the honor roll. I've tuned up the metal roof of a brick storage building in order to QNI into our 80 meter net. The Tx performance was awesome... but whoa!!! Definitely not as desirable as a Beverage on Rx.

3) Maybe this is obvious to everybody, but I'll comment on the array of VNA photo's you attached. The Smith Chart pretty much tells the whole story, and all that follow are simply different ways of displaying exactly the same information. VNA's measure reflection coefficient; the displays are simply computational results of the same measurement.

Bottom line... I'm convinced of your ability and knowledge of using a VNA to correctly and accurately tune antennas. Getting yours tuned precisely where you wanted it is certainly a bigger challenge than tuning up a cook book design. Try getting on the air an hour or so after dusk... and especially on weekends.... when hams actually make their presence known. Try answering a few of the stronger CQ's. You may be surprised at the gems that are borne out enthusiastic dumpster diving. LOL!



--
73

Gary, N3GO