Poor antenna performance?

[VK3BPN]

During last winter I built antennas for 6 and 2m both DK7ZB designs http://www.mydarc.de/dk7zb/ , the 6m antenna is a 3el 1.90m boom 5.7dBd and the 2m antenna is 5el 2.0m boom 9.0dBd. The antennas are up about 5m with reasonably flat take off in all directions.
The 2m antenna seems to work quite well but I feel the 6m antenna performance is well down on what I would expect for example our local 6m beacon VH3RMH is less than 40km away and it barely raises the S Meter. I am not judging the antenna performance solely on RMH but also against what other stations in the Melbourne Metro area are hearing and working in many cases I can barely hear.
I have triple checked the measurements of the antenna and matching stub and the SWR, which is good, the antenna is fed by about 12m of LMR240 coax. I expected to get reasonable results from this antenna so where is the problem, are these antennas duds? Is the antenna too low? What’s your thoughts?

Cheers Peter
VK3BPN

[VK1DJA]

Peter,

I have built a couple of DK7ZB beams, for 6m and 2m. I think they are performing close to the specs.

When you say your SWR is good, are you close to the 1.1 that is given on the DK7ZB website, or a bit higher?

Are they showing some directionality?

One drawback in the DK7ZB designs is that they are narrow bandwidth. My 6m beam, for example, is pretty deaf above 52 Mhz.

Dave VK1DJA

[VK4APN]

Which version of the 6m beam did you make? The 12.5ohm or the 28ohm?
I have modeled some of his designs using EZNEC and they have been absolutely spot on. I can plug in the dimensions and run them on EZNEC if you like. Let me know....
Paul vk4apn

[VK2JDS]

are they on the same mast? (spacing) how far away from metal roof/shed etc are they? and proximity to nearby trees or structures?

[VK4TU]

How broad is the SWR curve? If it's very wide that suggests that the coax has too much loss/is shot. Good SWR does not mean that the antenna is good, just that the power isn't being reflected back. The other question is has the rig gone deaf? Do you have some way of checking it?

73,
Joe

[VK4BKP]

Using VK3RMH beacon as a reference, rotate the antenna 360 degrees and note the lobes. If the largest lobe is directly out the front with other lobes including the back suppressed, chances are it's working. If the strongest lobe isn't coming out the front, or if the other lobes are not reasonably suppressed you have a problem. Note that you will get some spill off the back and sides but they should be down. How much depends on antenna design.

[VK3BPN]

Answers to some questions:
Dave VK1DA, SWR: 50.01 1:1, 50.2 1.1:1, 50.4 1.2:1, 50.6 1.15:1, 50.8 2:1. Directivity is about what I would expect for the size of the antennas.

Paul VK4APN, 28 ohm. Dimensions are exactly as shown.

VK2JDS: Both antennas are on the same mast about 1m apart and about 2.5m above the roof of the garage which has that silver insulation under a tile roof. The to the north where the RMH beacon is the 6m antenna looks over next doors roof by a metre or so with a huge pine tree just to the right of the track 2 blocks away.

Joe VK4TU: See above for the SWR curve and yes I understand your comments about the meaning of the SWR. The COAX is 3 months old. I took the rig Yaesu FT627R to work this morning and checked it on a calibrated service monitor and the S/N ratio at 50.10 is 0.13uV for 10dB. Its gain is also good 1uV = S2, 5uV = S5 so no problem there.

Kevin VK4BKP: Unfortunately I have PLI which is usually S5 to 7 so I would need to grab a time when there is no interference to do this check but the strongest lobe is definitely off the front and the directivity pattern appears what I would expect from a 3el beam.

Thanks for the suggestions hopefully we will get to the bottom of the problem

Cheers Peter
VK3BPN

[VK3SIX]

Hidetsugu Yagi attempted wireless energy transfer in February of 1926 with this antenna. Yagi and Uda published their first report on the wave projector directional antenna. Yagi managed to demonstrate a proof of concept, but the engineering problems proved to be more onerous than conventional systems.




I get the impression you expected a lot from the antenna and if it had been even a 4 element I might have made comment or a 5 or 6 but as a 6M antenna a 3 element yagi basically doesnt rate in the performance stakes, how can it? Its 1 element above the yagi UDA design that said if you put a shorter or longer parasitic element near a dipole you get some gain.
What were you expecting from a 3 el yagi?
You said you have 6DB.
Is that what you Measured?
The vswr readings you quote have nothing to do with its gain or directivity.
The s meter readings from 3RMH on whatever radio you are using are not empirical and mean nothing unless you have some reference calibration.
What is an s metre reading or a LED reading or a bar reading on a modern radio anyway?
How sensitive is your radio?
Whats its IF and RF gain?
What are they set at?





A35RK thought he had a great antenna The STEPIR then he put up a 4 element yagi and he has a better antenna?
Or is the first a worse antenna?

Could I suggest you at least put up a dipole and compare?

Switch between both and at the far end get a reading and dont tell the far end which antenna you are using.
You might just find that the 3 element yagi is a dipole with wings.


2 cents.

[VK4APN]

Loaded the design into EZNEC and received exactly same results as published....fairly typical results for short 3 el yagi. Others have some worthwhile advice/suggestions to follow up.
Good Luck
Paul vk4apn

[VK3SIX]

EVOLUTION of PARASITIC BEAM ARRAY

QST 2008/FEB PAGE 57 explores all the developments since inception in 1920.
In 1928 a 10M beam was built.


Basically DIRECTORS spaced 3/8th wavelength apart.
DE was a FULL WAVE
REF WAS 1/4 WAVE BEHIND.


Fed by open wire feeder.



SHINTARO UDA used 1 reflector and 7 directors in his first model.


On the roof of Tohoku University on 68 MHZ.


Free space gain was 13 dBi

Front to back 20db.



Close spaced beams started in 1937 USA.

G5PP made a 2 el 56 meg beam in 1936


Great reading

[VK4ABW]

During last winter I built antennas for 6 and 2m both DK7ZB designs http://www.mydarc.de/dk7zb/ , the 6m antenna is a 3el 1.90m boom 5.7dBd and the 2m antenna is 5el 2.0m boom 9.0dBd. The antennas are up about 5m with reasonably flat take off in all directions.
The 2m antenna seems to work quite well but I feel the 6m antenna performance is well down on what I would expect for example our local 6m beacon VH3RMH is less than 40km away and it barely raises the S Meter. I am not judging the antenna performance solely on RMH but also against what other stations in the Melbourne Metro area are hearing and working in many cases I can barely hear.
I have triple checked the measurements of the antenna and matching stub and the SWR, which is good, the antenna is fed by about 12m of LMR240 coax. I expected to get reasonable results from this antenna so where is the problem, are these antennas duds? Is the antenna too low? What’s your thoughts?

Cheers Peter
VK3BPN

Peter. There could be other factors involved. Reflections of buildings, trees, etc are all trying to cancel or reinforce the signal. I remember building my first long boom yagi for 6m in 1990 and discovering that a tree (about 100ft away) affected the swr! Even though my antenna was 60ft off the ground.
More height in your situation might provide some improvement. Experiment with different heights.
As mentioned, don't expect too much for that amount of aluminium. Your situation might not permit much more in the air but there are some very good designs around these days. Or you could use a preamp if you want that needle to move.

best of luck
Gary