denpa ga ii tokoro made aruitara, tomodati ni renraku dekimasita.

It is a noun phrase built with a clause modifying ところ.

• 電波がいい = the signal is good
• ところ = place

So 電波がいいところ literally means a place where the signal is good.

A key point is that Japanese does not need a word like where here. The whole clause 電波がいい comes before the noun ところ and describes it.

In 電波がいい, 電波 is the subject of いい.

So the structure is:

• 電波がいい = the signal is good

Here, が is the natural particle because it marks what is good. Then that whole clause modifies ところ.

If you said 電波はいいところ, it would sound different and would not work as naturally in this structure. が is the normal choice in relative clauses like this.

Here, まで means up to, as far as, or to.

So:

• 電波がいいところまで歩いた = walked as far as a place where the signal was good

This is not the until meaning of まで. It is marking the endpoint of movement.

Here, たら is better understood as when or after rather than a hypothetical if.

So:

• 電波がいいところまで歩いたら、友達に連絡できました。
• When/After I walked to a place with good signal, I was able to contact my friend.

This use of たら is very common when one action happened first and then another result happened after that.

It can mean if in other sentences, but in this one it describes a sequence of events.

Because the sentence is emphasizing ability/success, not just the act itself.

• 連絡しました = I contacted
• 連絡できました = I was able to contact / managed to contact

Since the sentence talks about walking to a place with better signal, the important idea is that contact was not possible before, but became possible once the signal improved.

Also, 連絡 is a verbal noun, so:

• 連絡する = to contact
• 連絡できる = to be able to contact

With 連絡する, the person you contact is usually marked with に.

• 友達に連絡する = contact a friend

So に marks the target/recipient of the contact.

Using を here would be incorrect.

Yes. Japanese often omits subjects when they are understood from context.

So the sentence does not explicitly say I, but the natural understanding is something like:

• I walked to a place where the signal was good, and then I was able to contact my friend.

Depending on context, it could technically be we, he, she, and so on, but I is the most natural default in many learning examples.

Yes, in this sentence it most naturally means a real physical place.

Because the verb is 歩いた (walked), the sentence strongly suggests physically moving to another location.

So here ところ is simply place/spot/location.

Literally, 電波 means radio waves, but in everyday Japanese it often refers to signal or reception, especially phone signal.

So in this sentence, 電波がいい means something like:

• the signal is good
• the reception is good

It does not sound overly technical in this kind of context.

Because Japanese normally puts modifiers before the noun they describe.

In English, you say:

• a place where the signal is good

In Japanese, the equivalent structure is:

• 電波がいいところ

So Japanese relative clauses come before the noun, and there is no need for words like where, that, or which.

The comma separates the たら-clause from the main clause:

• 電波がいいところまで歩いたら、
• 友達に連絡できました。

It helps show that the first part sets up the situation, and the second part gives the result.

It is not absolutely required in every case, but it is very natural and makes the sentence easier to read.