70cm folded dipole

[6meter]

I do like the jpole idea however, I wanted a horizontal antenna for the weak signal portion of the bands. What if I lay the jpole on it's side?

A dipole for weak signal work on 2m or 432 will be a rough way to go.

Antenna height, low loss coax and antenna gain are very important.

Look at something like the quagi
The Quagi antenna

IK1HGE 432MHz 8el. Quagi Antenna

They are fairly easy to build. For fun I built a quagi for 222.

 

[The DB]

I have not come across the terms "voltage loop" and "current loop" before so doing some research.

Google for one is useless, nothing seems to fit antennas or transmission lines, and if I add "feedline" or "antenna" to the search it refers to loop antennas only...

I did note that there are index entries in the 15'th and 16'th editions of the ARRL antenna books. These entries did not exist in the 14'th edition, and were gone by the 18'th edition. (I don't have a 17'th edition... yet...) When I looked at the same chapter in the 18'th edition, which has the same name, that whole section has apparently been rewritten and shortened.

When I read that section in the two books that have it they are referring to voltage and current loops in the antenna itself, not the feedline. I think this forum typically uses the term "node" in place of "loop", however, "peak" would also do nicely.

I am curious as to how you are applying this to a feedline as in a feedline the forward voltage and current are in phase with each other, and the nodes or peaks (or loops as you called them) are not stationary. When working with the reflected waveform, they are 180 degrees out of phase, which means the voltage and current peaks, while opposite polarities, still happen at the same points in the waveform.

Or am I completely misunderstanding what you are referring to by "voltage loop" and "current loop"?

Sorry for the hijack Leland...


The DB

 

[Leland]

Lets say that my dipole is 72 ohms at the feed point, my coax is 50 ohms so how do I take care currents on the feed line with out creating to much loss? Someone suggested a 1 wavelength 1:1 50ohm coax balun around a 1 inch pvc form. Would this work? I dont have a tuner for these frequencies so I'm rather hesitant.

 

[hookedon6]

Lets say that my dipole is 72 ohms at the feed point, my coax is 50 ohms so how do I take care currents on the feed line with out creating to much loss? Someone suggested a 1 wavelength 1:1 50ohm coax balun around a 1 inch pvc form. Would this work? I dont have a tuner for these frequencies so I'm rather hesitant.

a 1:1 balun won't change the feedpoint ratio at all.

the impedance of a dipole changes as the height above ground changes,.... raising (or lowering) the ends will change the impedance.

 

[hookedon6]

...I am curious as to how you are applying this to a feedline as in a feedline the forward voltage and current are in phase with each other, ...
Or am I completely misunderstanding what you are referring to by "voltage loop" and "current loop"?

Sorry for the hijack Leland...


The DB

actually, they are 90 degrees out of phase

go here: http://us.yhs4.search.yahoo.com/r/_ylt=A0oG7qdFCKpRfhoALQAPxQt.;_ylu=X3oDMTByMTNuNTZzBHNlYwNzcgRwb3MDMgRjb2xvA2FjMgR2dGlkAw--/SIG=11m6qvslc/EXP=1370126533/**http%3a//www.eham.net/articles/18584

this is how antenna analyzers can "compute" the correct elect wl of coax (as was previously mentioned in this thread). even a cheap field strength meter will show nulls and peaks if it its moved along the run of coax.

 

[The DB]

actually, they are 90 degrees out of phase

go here: http://us.yhs4.search.yahoo.com/r/_ylt=A0oG7qdFCKpRfhoALQAPxQt.;_ylu=X3oDMTByMTNuNTZzBHNlYwNzcgRwb3MDMgRjb2xvA2FjMgR2dGlkAw--/SIG=11m6qvslc/EXP=1370126533/**http%3a//www.eham.net/articles/18584

this is how antenna analyzers can "compute" the correct elect wl of coax (as was previously mentioned in this thread). even a cheap field strength meter will show nulls and peaks if it its moved along the run of coax.

To start, your link tells me that it is not authorized by Yahoo. And in the tab it opens it says "Yahoo! - 410 Gone".

Challenging your 90° claim...

The total voltage (or current) at the load at any instant of the sum of the voltages (or currents) of the forward and reflected waves. The in-phase reflected voltage wave is verified because the sum of the two voltages at the load is double that of the forward voltage. And since the two current waves add to zero at the open circuit load, the generation of the reversed-polarity reflected-current wave is also verified. The phase angles, θ, of the reflection coefficients at the open circuit load are therefore 0° for voltage and 180° for current.

Near the end of the next paragraph referring to a short circuit load:

The phase angles θ, of the reflection coefficients at the short circuit load are therefore 180° for voltage and 0° for current.

Also...

The maximum of the reflected voltage is positive when the current maximum is negative, and vice versa, which means, as stated earlier, the reflected voltage and current are always 180° out of phase with each other. But does the phase really matter here? Indeed it does--there is no other relationship more important to wave propagation on a transmission line! The 180° out-of-phase relationship is directly responsible for the development of the standing wave, as well as for the change in the input impedance of the transmission line from Zc to a complex value when the load terminating the line is mismatched.

Maxwell directly addressing some writers 90° phase angle claims.

It is of interest at this point to be concerned with the nature of the power in the forward and reflected waves. Some writers contend erroneously the the voltage-current phase relationship in the reflected wave is 90°. If this were true, then the reflected wave would contain only reactive volt amperes, and no real power. The evidence presented above disproves this contention since we have seen that the voltage-current phase relationship in the reflected wave is 180°, not 90°. And certainly we will agree that if real power is conveyed in A if Fig 3-1, it is also real power in B, or C, even with reversed current-meter or voltmeter terminals. We will agree that real power P equals EI cosine θ, in which cosine θ is the power factor. It matters not weather the phase angle is 0° or 180°, because cos 0° = 1 and cos 180° = -1. This simply denotes the polarity difference discussed above.

The figure 3.1 mentioned above is here:

That is from the "Another Look at Reflections" .pdf file which is freely available on-line, and matches the Fig 3-1 in the book. Also, the above quotes are also available from that .pdf, but the pages are numbered differently. The first quote is from pages 16 and 17. The second quote is on page 17. The third quote is on page 18, and is different in the .pdf and the book. The fourth quote is on page 19, and is also different in the .pdf than the book.


The DB

 

[Mudfoot]

This is the link he posted.

http://www.eham.net/articles/18584

 

[The DB]

This is the link he posted.

http://www.eham.net/articles/18584

Thanks, I'll read through it tonight.


The DB

 

[The DB]

Read through it... I've read that or something very close before as it sounds very familiar.

I looked up more info on the "Lecher line" from other sources, and those read as this one...

http://pec.sjtu.edu.cn/ols/P3/P3731_E.PDF on the same subject includes the following texts:

If the wires of the Lecher line are short-circuited at the end, then the voltage U is zero. A reflected wave arises with a phase shift of 180° with respect to the incoming wave.

Then below that...

The voltage between the wires is associated with the charge distribution along the wires. The displacement of the charges leads to current I in the wires which propagates as a wave. There must be a permanent current at the jumper. The incoming current wave is therefore reflected without a change in phase:

Looking at the next section...

The situation changes, when the end of the Lecher line is left open. Now there is a permanent voltage at the end; that is, the incoming voltage wave is reflected without a change in phase, whereas the current in the open end is always zero, the incoming current wave being reflected with a phase shift of 180°.

As a matter of fact, the only source I have seen that has any indication that the Lecher line shows that standing waves are 90° (as opposed to 180°) out of phase is the eham link given as a source above...

This .pdf also has a graph that shows that the voltage and current nodes happen at the same points along the line.


The DB

 

[yo2mgy]

good
Can someone help me with a color scheme Arrow Antenna 146/437-10?
Scheme to send e-mail: yo2mgy@yahoo.com
Thanks
yo2mgy

 

[Leland]

Ok, I built the antenna with interchangable elements. After using an MFJ 269 I found that a perfect 1:1 could not be found on any frequency in the 2 meter band. Also, I made elements for 70 cm that turned out much lower on the swr meter at 1.3:1. How would using ladder line be different than the coax I'm using now? And if I did, how do I adapt the ladder line the SO239 terminal?

 

[Captain Kilowatt]

Lets say that my dipole is 72 ohms at the feed point, my coax is 50 ohms so how do I take care currents on the feed line with out creating to much loss? Someone suggested a 1 wavelength 1:1 50ohm coax balun around a 1 inch pvc form. Would this work? I dont have a tuner for these frequencies so I'm rather hesitant.

This is your first mistake. The feedpoint impedance of a folded dipole is 300 ohms. Not at all near 75 ohms. A normal straight dipole has a feedpoint impedance near 75 ohms. You generally use a QUALITY 4:1 balun at the feedpoint of a folded dipole to transform the impedance down to something in the order of 50-75 ohms to match your coax cable. A simple coaxial balun is easy to make.

http://www.k7mem.com/Electronic_Notebook/antennas/yagi_vhf_feed.html

 

[Leland]

Ya, I changed the plans in the middle of the project. It started out as a folded dipole and ended up as a regular dipole. Is that coaxial balun less lossy than a toroid balun? I will be feeding a yagi with a folded dipole. And my other question from the last post, How do I match ladder line to the radio. Would this be done with a balun as well?

 

[Captain Kilowatt]

Things start to get real messy in a hurry using ladder line and baluns on 70 cm. You end up introducing more losses than you gain. If you are feeding a yagi then forget about what impedance a dipole or a folded dipole is. The impedance of that dipole will drop significantly once it becomes the driven element of a yagi. You need to know the antenna feedpoint impedance and not the impedance of a stand alone dipole before you start figuring out how to feed it.

 

[Leland]

Things start to get real messy in a hurry using ladder line and baluns on 70 cm. You end up introducing more losses than you gain. If you are feeding a yagi then forget about what impedance a dipole or a folded dipole is. The impedance of that dipole will drop significantly once it becomes the driven element of a yagi. You need to know the antenna feedpoint impedance and not the impedance of a stand alone dipole before you start figuring out how to feed it.

So, build the antenna first and feed it last. What is a typical feedpoint impedence for a yagi with a folded dipole as the driven element? Or are there to many variables to say? I guess I should find some plans and get building.